The half rollover touch is an advanced version of the basic rollover but instead of rolling entirely over and then plant the foot on the opposite side, you basically stop halfway and use the outside of the foot to move the ball the same way you came from originally.
You can use that move when you have a defender in your back to shield the ball, you want to deceive the defender moving him to one side, but then move quickly back to the other side.
It's a quick way to change directions with the most control because you use the bottom of the foot.
It's not a high speed move because using the bottom of the foot usually means or can mean that you have to slow down a little bit to keep the balance, but if you do it quickly, accelerate afterwards, it's a nice way to get the defender one way or another.
That's the basic idea of doing the half rollover touch.