NONE: You aim your swing manually with the blue bat cursor. This option will have the most realistic results and best hit variety because of the true "bat on ball" physics. 

ASSIST: The CPU helps you make contact with the ball. There are 5 assist levels, with level 1 having the smallest amount of assist. The hit variety will suffer depending on what level you're using because the higher you go the more the CPU has a tendency to center up on the ball & hit line drives. However, those who are having difficulty with normal aiming or are using a VITA (with the smaller analog stick) may like this option. If you use this option it's recommended you start with level 1