It's the body language that gives you away, and you succeed in combat when you don't think, but do.
'Muscle memory', as people call it, is way faster than any thought process. Thus, we practice. And practice. And practice some more. And eventually, we think less and do more, and our opponents say "what the hell was that?"
It's a good place to be. :-)
It's oh so very true. Every now and then at fencing, I've found myself there. My opponent attacks, and a few seconds (or fractions thereof) later, I find myself standing with my point on their chest, and mentally replaying the previous moments in time to figure out what it was that just happened. And when I say "fractions thereof", it can really be that. Watch an olympic fencing bout sometime (or a world championship bout, like this one from 2006). Fast, eh? Sometimes there are a few seconds (or minutes!) of back and forth with no-one really attacking, but when an attack starts, it's often all over very quickly.
Having fenced for about five years now (or is it six?), I'm really starting to get the hang of it - the figuring out what happened thing, that is... I still generally suck at fencing! But, while I usually forget the French terms for the actions & reactions, I can often explain to my opponent something like this:
"Ok, I initially did a straight-line feint, you did a parry quarte, I disengaged, you followed with a counter-quarte, I disengaged again for the hit."
"Quarte" is a guard position where your forearm is across your body, "counter-quarte" is a circular parry intended to move your blade beneath your opponent's and push it off-target, ending in a quarte guard position, a disengage (there's a french term for this, but I can't remember how to spell it!) is, in this case, me moving the blade out of the way before the parry makes contact.
I'm finding, these days, that when I'm "in the zone", as it were, many of my actions are "instinctive", in that I don't think through what's happening at each point in time. Occasionally (and this is the bit that makes me quite pleased with myself!) I find that between points, I think "Hey, if I do X, then my opponent does Y, which means I can then do Z and score a hit", and then it happens just like that!
Then I fence someone really good, and I get put in my place. :-P