I didn't test full release but would it make any sense for the button to be inactive if you haven't hit the trigger yet? I also didn't understand how the percentage number works for turns taken, or for the health/endurance difference if it's saying you're at x% of opponent's stat, or x% less than (as in is the calculation PC / OP <= x or (OP - PC) / OP >= x).
Oh and is the power of your full release based on the percentage it triggers at? That seemed like how it would work but I don't remember if the help text specifies it.