One of the available upgrades in Batman: Arkham Asylum is the "Critical Combo Strikes":
Critical Combo Strikes Doubles the power of combo hits and perfectly timed attacks will count as 2 combo hits.
After unlocking it I do occasionally get a "critical!" message, but I'm not really sure what triggers them. What is a perfectly-timed attack, and how can I execute one?
To get the criticals you need to time the button press for your next attack just as the current attack lands. There's a small window around the time so it can be slightly after or before, but not a lot.
Spend some time in the first Challenge room... When you're doing it right, it has a very fluid feel to it. Once you get the rhythm down it's almost hard not to get the criticals.
You do nothing special to hit a critical, a critical hit is just a hit. There is one condition that makes a hit critical - you should NOT attempt any other actions between this hit and the previous hit. So, if you managed to chain a 20 criticals (which is not that difficult once you get the hang of it), you pressed the hit button 20 times during this time and nothing else.
The usual way to lose a critical is to try executing next hit before animation for previous one finishes. Tip: try to aim at the guy that is furthest from you, dashing from one part of the room to another and back. This spaces your attacks and makes it easier to get the feel.
From what I've been able to tell, its about relaxed timing. You have to time your attacks just a bit slower than you'd expect to get the Critical Combo Strikes to work. The idea is ment to deter you from button mashing. You still have to be pretty quick to get the combo, but you want to draw it out as much as you can. I think the best suggestion would be to practice a bit in the challenge rooms to get a feel for the timing.
