In the game "En Garde!" - How do optional routines work?
If my opponent starts with a Furious Lunge, I believe our sequences would go:
  
  -L-X-X-C-X-X-
  -P-R-P-X-L-X-


No: your written sequence would be P-P-X-L-X-X-S... (to 12 actions)

The optional routines kick in when the duel is resolved.

So, when you see that your opponent's first action is a lunge, which is stopped by your Parry, you may replace your next routine with a Riposte. If you do so, the sequence that is executed is P-R-X-L-X-X-S...

Note that it replaces the next routine. If your opponent went X-L-X-X-S... to start with, your second Parry works and you may then Riposte. However the R replaces the X-L-X routine (padding with rests), so you end up executing P-P-R-X-X-X-S... As a Riposte does less damage than a Lunge, you might well prefer not to Riposte in these circumstances.

Personally, I don't think the other optional routines are particularly useful. There are some very specific circumstances where they can be used to avoid damage from an opponent's attack that you can see coming an action or two in advance, but this is often at the cost of sacrificing an attack of your own.

In a postal/turn-based game like mine (www.pevans.co.uk/LPBS), the optional routines have to be used as conditional orders in a player's duelling orders: "If the first parry succeeds, then riposte; if the second succeeds, do not Riposte" for example. This can get very complicated...

The duelling rules work much better face-to-face - as originally intended - as there's the element of out-guessing your opponent.