As a professional tennis instructor, raising many ranked juniors, I can honestly say, the straight arm forehand is more volatile than the double bend can ever be, but it is much harder to get used to, because it requires all adjustments to ball hight and distance with very fast footwork rather than arm adjustment.

Federer has one bend in his arm to racket when he strikes the ball. It isn't his elbow, it's his wrist. What this alows him to do is to utilize his chest more than tricep for power. The Chest is a much larger muscle, therefore more powerful. Nadal also has a contact point where he does not use his tricept to generate pace. Both strike the ball right as the wrist about to snap foreward, in a contact zone far infront of their body. They have to hit there, or there wrist will not be in the proper position to hit with pop and penetration. The key is to utilize the forearm and wrist rotation at the fulcrom of the swing. The very farthest part of the swing, the very midpoint, for maximum racket head speed.

Both Nadal and Roger Federer use head light rackets because it compliments this unique style of single bend forehands. It is complimentary, because it allows the player to whip the racket around the back of the ball and across the shoulder without using the elbow or wrist to generate pace. The sling shot motion of both players is unique to the tour, but their forehands are by far the most deadly, and the most consistent because of the focused muscle groups, and elimination of unecessary muscle groups.

If you get anything from this, they use formula, a formula with head light rackets such as Federer Racket Wilson K Six-One Tour 90. Which is handle heavy allowing the momentum of the arm to snap the racket foreward instead of the wrist and forearm.