To keep it simple, "indes" means "just" or "while" as in "just/while something happens". Ergo, someone acts *while* some other action (the opponent's, usually) is happening, as opposed to stopping an action and *then* starting a new one.
Say, someone is trying to whack you in the left side of your head with a blow from above (right oberhau/zornhau to your obere linke Blöße). Unless you have cunnigly decieved him in some way, he has the initiative and you don't (he's in the Vor, you are in the Nach). Now, if you stepped to your forward right, leaving the line of attack, opening a new line yourself, and whacked him with a strike to the left side of the head *while his strike was still travelling towards you*, you'd be acting "indes" (while). In this case, attacking him *while* making sure his attack is no longer a threat.
Literally, indes means to do something while the other guy is doing something, as well. It is implied, however, that you actually act in the offensive while the other guy is, as well. This happens when you are at a disadvantage, the other guy has the initiative. Instead of just defending (which would literally be indes, as well), you attack him in a way that keeps you safe at the same time.
The five cuts, duplieren, mutieren, winding... the whole system is built on the idea of hurting someone who is on the offensive. That's the Art, because you don't need any art if you are faster, anyway. Always remember that the system described by Ringeck, Danzig et al. is just plan B. Plan A is always to step up, kill the dude by hitting him first, hard and not stop hitting until he's miced meat. If that doesn't go quite right, do what the book says.
