The reason you turn sideways is so you can make a full overhead motion. Keeping your finger pointing up until just you make contact with the ball is so that you can keep your head up. If you drop your head while hitting an overhead, your hitting shoulder will rotate down, so when you make contact with the ball you will be hitting in a downward motion, which will cause the ball to hit the net. So, to make the proper contact and wrist snap over the ball, you need to keep your head up.
Overhead shot opportunities always result from an opponent’s mistake, generally a lob that is either too low or too close to the kitchen. Overhead should be hit only when the ball is high enough that you can reach it with the center (sweet spot) of your paddle using a full arm extension. A volley should be used on any ball below that height. Hitting an overhead on a lower ball will usually result in a ball hit into the net.
The overhead is a forehand shot and, therefore, a forehand grip is necessary. The continental grip is recommended because it naturally allows the paddle to slice through the air during the swing and then to open in order to hit the ball squarely at the contact point.
A short lob is hit softly and gives a defending player sufficient time to both fix the grip and gain the proper body position for the overhead shot. As soon as the shot is recognized as a lob, the player should turn his shoulders and position his body and feet into a sideways position. If necessary, the player should move back in this sideways position to get under the lobbed ball.