There's been a tutorial in game for Aftertouch since launch which explains the mechanic - the tutorial will explain aftertouch as per the control scheme you have selected. For pro it tells you quite clearly that the inputs are for longer/shorter length.
I must have missed it, how I do not know but, I did ask if there was a simple explanation on the forum. The answer is none conclusive in many videos on you tube which eluded me to think there was no tutorial.
It just is that the release point changes - so the ball gets pitched up or shorter based on exactly when you release the ball. 'Spray' just refering to that it's not something you control the exact amount of.
I am wondering if it can be 'tightened' up as it is just too sensitive.