What is nATB?  It is a modification to the FF6 Active Time Battle mechanic that forces the ATB meters to pause while an animation is playing.  nATB makes differences in the speed stat more significant and can increase the game's difficulty by giving monsters more opportunity to attack.

Credit:
Think0028 (original nATB code, forced the game into permanent Wait mode.)
C-Dude & RoSoDude (modified Think0028's code to get Active mode back up and running.)