From high speed video I'm not seeing a clear pattern yet that shows what is happening with a high takeback. But some players such as Del Porto, Gulbis and Florian Mayer raise the elbow high (upper arm) and it appears to be adding pace.
If some players have learned how to use a high elbow maybe they are using shoulder flexion to initially rapidly pull down the upper arm. However, this is not apparent to me in most videos of forehands.
The reason that it would probably be used by some players in their forehands is that the largest muscle attached the arm is the lat. It can pull the upper arm bone, the humerus down very forcefully as in a pullup.
TennisOxygen has some very nice Youtube videos on take backs for the pros and how high they are. Most are listed under user name "xstf".
Federer, Nadal & Djokovic's and more similar forehand 1st move.
Look for videos comparing the forehands of players.
What high speed videos show more clearly is that body rotation contributes to FH pace.