This strategy is designed to quickly and cheaply kill/conquer a neighbour who is some turns behind in military technology in any kind of ffa/Ironman game. Like with any Ironman/ffa strategy its not something you decide to do at the beginning of the game, but something that needs specific circumstances to become a viable option, which I am going to explain as well.
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The General Idea[/SIZE][/U][/B]

The Idea is to upgrade 20-30 Knights to Cavalry once they become available and add some quickly slaves once to those and then rush in with about 30-40 Cavalry on an opponent who doesn´t have Riflemen yet. Having a two-moving stack of superior units then allows you to take down your opponent within 3-4 turns so he can´t put up any further resistance.
The other way this can be used, is to only raze 2-3 cities of someone in an Ironman/ffa game to take him out of the race for the lead.
Upgrading a Knight to Cavalry costs 68 gold on fast speed, so upgrading those Knights will cost you about 1500 gold. With the usual state of economy at that point of the game, this is three to four turns of money saving. Since you can upgrade your units after having moved 5 tiles on roads, you can have a Cavalry stack of considerable size in position to move in within four turns after researching them, which is possible no other way.
The second big point of this strategy is, that its difficult to notice. You can build the necessary amount of Knights over a long period of time, making it look like you only build them to have some defense army (which often actually is the case). This doesn´t cause a considerable rise in the power graph, especially not the sharp ones that are a sure sign of approaching war. The real rise in power only comes when actually upgrading to Cavalry which is done at the end of the turn you go into position to move in. Thus your opponent can only detect the rise in power in the very turn you are moving in on him. Of course there are more subtle hints something like this might be coming, be he has to monitor your moves rather closely to find you out.
This should already make clear, that is tactic doesn´t work, when you are playing with some kind of diplomatic agreements, that force you warn your opponent of your attack.

[B][U][SIZE="3"]Necessary Circumstances[/SIZE][/U][/B]

To be able to apply this strategy you need an opponent that reaches Riflemen about 8 turns after you reach Cavalry. This gives you enough time to wreak havoc on his land and economy so he never reaches them or if so, cant build enough anymore to put up considerable resistance. Note that your actual tech advantage might have to be bigger, because you need Military Tradition too, which he doesn´t need for Riflemen. On the other hand it might be ok to be smaller if your opponent doesn´t research for Rifling, but e.g. for Democracy instead.
Further it is of great advantage to have a long border to him, so you can quickly reach as many cities as possible. The important consideration here is, that you have to be able to take out his production capacities within about four turns, since your primary advantage is surprise and agility.
Further of course you need the Knights to upgrade. Often you simply build them anyway and only later decide to do the Cav Upgrade Rush. Or otherwise you have to plan ahead a little more produce them in time.
The general military level of the game shouldn´t be too high either. If everyone is running around with 60 unit stacks anyway, this can´t do any harm of course. Obviously you should also check the unit level of the one you want to attack in particular.

[U][B][SIZE="3"]The Advantage[/SIZE][/B][/U]

This strategy circumvents the two major problems of fighting a war in Ironman/ffa: Having to build (=slave) a huge amount of units very quickly hurting your economy and war weariness slowing it down even further during the war. With the Cav Upgrade Rush you basically only loose the turns for saving money. Since you can build the Knights over a very long period of time, this doesn´t do considerable harm to your economy. And since your war will end rather quickly and be fought with comparably small amounts of units war weariness doesn´t become much of a problem.
Also this is about the only way to fight a war in industrial age, because you can´t attack anyone who will draft Riflemen in defense.
And lastly its something you can easily blow of, if you notice your opponent is building too many units, reaching Rifling too fast or for whatever other reason, because as long as you don´t actually spend your saved money on upgrading, you havent hurt yourself at all and can use it for turning your science slider back up to 100% for some turns. Most likely you also forced several opponents into going for Rifling too, so they don´t gain an advantage by having researched economy techs while you went for military ones.

[B][U][SIZE="3"]The Tech Path[/SIZE][/U][/B]

This one can be summed up pretty simple: get Rifling and Military Tradition as soon as possible. However there are some small tweaks making things easier. The optimal version is winning the race for Liberalism taking Nationhood for free. Then tech the Machinery - Guilds - Banking (and possibly Economics if the free Great Merchant is still available) path so you can start building Knights, then research Printing Press. So far everything is looking normal. Only then you have to make the final decision to get Gunpowder, Rifling and Military Tradition which also allows someone who is monitoring your tech path to pick up hints of what you are doing.
You might also tech guilds earlier, because you need units in Medieval age for defense, this is all right giving you more time to build knights, however making it more unlikely to have the necessary tech advantage, because probably someone else will grab Liberalism first, but that doesn´t have to be the case.

[B][U][SIZE="3"]Execute The Kill[/SIZE][/U][/B]

When actually attacking first of all you have to draw together your stack somewhere, if you havent already done so. Important is to make it as difficult as possible to detect that your army is moving somewhere for someone with spies in your territory. If your Knight stack is already sitting in the middle of your empire this is perfect, so best draw it together there inconspicuously in time, making it look like the well placed defensive army. Now time moving your stack towards your opponent with getting enough gold to upgrade it, so you can move it into attack position and upgrade at end of turn, the very turn you get enough money. (Note that you can only move 5 tiles on road that turn, because you need move to be able to upgrade.)
Then move in on your opponent at the end of the next turn at one or more places, if possible on double hit positions. For the success of the whole attack it is vital, that you apply the rules of moving at end of turn and reaching double hit positions Imhotep described in his article on Attacking in Civilization Multiplayer.

[B][U][SIZE="3"]Detecting what´s coming[/SIZE][/U][/B]

So how to notice someone else is planning to do this to you? Well, simply check for the ingredients. Constantly check your opponents techs either via the Foreign Advisor or Espionage, as you should be doing in anyway. If you notice him heading for Rifling before Democracy something is foul, because that doesn´t make any sense, except if he wants to attack in some way. Use spies to check for a Knight stack and last of all check for him saving money after he is able to build Cavalry, which is the definite telltale sign.

[B][U][SIZE="3"]Counterstrategy[/SIZE][/U][/B]

If you actually know whats coming, counteracting it is rather simple. If you can get Riflemen in time do so and get a few as soon as possible, which should swart your opponent from his plan. If you can´t get them in time, get as many Pikemen as possible together with a considerable amount of catapults. Hit the cav stack with your catapults and stack up the pikemen in your cities. Your opponent could still ruin you, but that doesn´t help him anything if you are that far behind anyway and enough pikemen will make killing your cities too expensive for him.

[B][U][SIZE="3"]Conclusion[/SIZE][/U][/B]

The Cav Upgrade Rush provides you with a very flexible and easy means of conquering an opponent after the expansion and economy buildup phases of an Ironman/ffa game. Its main advantage are the low impact on your economy and the possibility to change plans quickly to constantly adapt to the ever changing global situation in an Ironman/ffa games. When you are leading a game by tech, you can use it to conquer another player, securing your victory or throwing back an opponent that is close behind you, possibly gaining by having more land by razing him some major cities. The latter is only possible in Ironman though, because without always war, you are stuck with a war and some war weariness afterwards.