Long Knee and Short Knee

hey guys, i was curious as to the proper way to execute the long knee and the short knee. under what circumstances would you want to use either of these? also, where are your hands while performing the knee? and what is your final position? do you kinda 'fall' into the opponent, and push them aside, or do you step back into your original position and stance?

Thrust you hip forward then throw you knee. Your opposite hand is in front of your face and adjacent is droped.