Hitting high forehands is a difficult shot for a lot of players, but the mechanics of how to hit a high ball are largely similar to handling a normal forehand. Instead of dropping the tennis racket down as you would with a normal forehand, keep your arm and the tennis racket higher up on the high ball. You set your arm and racket in your hitting-arm position, and you then swing forward to your contact point on a higher plane than you normally would.
At about 40 seconds into the video we have a shot of Oliver Akli hitting a normal forehand and a high ball side by side. On the left is the high ball, and the right side is the normal forehand. You can see that when he gets the racket back and prepares to swing forward on the high ball, he leaves his arm and the racket up high. He sets his hitting-arm position, just like on the normal forehand, but then swings through the tennis ball on a higher plane.
We’ve put a line along Oliver’s swing path to make this easier to see. The motion on the high ball is much less of a loop than the normal forehand, and you can see that he keeps the tennis racket and arm much higher on the high ball. Note, however, that the other fundamental mechanics of the two swings are identical. He still establishes his hitting-arm position as he prepares to swing forward, and he maintains the hitting-arm position through contact.
So in sum, to hit the high ball, Oliver has taken the same hitting-arm position he would use for a high ball and simply raised his arm at the shoulder.