Straightening the arm, mean to minimize the elbow joint hinge action. The result is a shoulder pivot swing ... slower but heavier at contact.
At club level is very hard to keep tha ball in play with a straight arm forehand ... maybe because it is harder to spin the ball with the arm straight.
I imagine that Federer is able to add to that shoulder pivot swing (power), a wrist action, keeping the elbow angle firm just before the contact and relaxing the wrist joint to have a whip action (spin), according with coach Dougherty that explain: a whip action needs a soft segment attached to a solid segment ..
... what imagination ... !!