Pre-Attack is the mechanical noise that a piano makes before the hammer strikes a string. Not all midi controllers (if any) can properly playback pre-attack because on a midi controller the pre-attack sample is triggered at the bottom of a key press instead of at the onset of a key strike like on a real piano. This video explains more:

Production Grand includes Pre-Attack to give the user an authentic piano experience. Pre-Attack is not without its issues. The main issue is that the pre-attack sample length differs by velocity and by position on the keyboard (higher or lower). A low velocity note would naturally have a longer pre-attack time as it takes longer to play the key. This makes playing with pre-attack enabled rather challenging from a latency stand point.

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