Some quick points on the tennis forehand:
Unit turn- Simply means turning the shoulders sideways towards the forehand direction.
Both hands should be on the racket when you are doing the unit turn.
Separating the hands too early and wide apart causes the shoulder to remain square to the court.
When I do my unit turn, I tend to think that my non hitting hand is helping to lift the racket up.
I also use my non hitting hand to push my wrist back so that it is flexed.
When both hands are separated, they should still be on the forehand side, especially the non hitting hand (See Federer's left hand below).
When do you split both hands?
Even if you managed to do the unit turn, separating both hands too early and too wide apart may cause your timing and rhythm to be off.
The separation of the hands and the striking of the ball should be timed well and it is a smooth fluid swing.
Think of the rhythm, slow.... FAST! Slow backswing and fast contact to follow through.
The reason why I wrote this short post is because I happened to see many tennis players separating their hands very early and causing their timing and rhythm to be very off, resulting in poor shots. So I hope these few critical points can help them to improve their technique further.