I believe the default for IS Boxer, is to execute the step/action on key release.
But you can change this, via a drop down box.

Or you can have each step advance to the next stage on key down and key up actions; naturally the GCD might get in the way for many actions, it really depends on what you are doing. IWT (down) and DPS Key (up), would not interfere with each other, since IWT is not on the GCD. Similarly IWT (down) and Down Arrow (up), to break movement, would not "tangle" and you could use this to recover from a fear bomb and control how closely you run towards the target by timing your release.

Before people go on about up/down being one action, like they derailed the last thread, just remember Blizzard put the keyboard API in the game, which allows addons to register distinct actions on up and down; this is an option Blizzard has put into the game.