The whole upper torso does not turn back down like it is a separate thing. We are unwinding from ground up like a rubber band. On the downswing, the body essentially is unwinding in the opposite direction. The right or back elbow will open up on its own due to inertia. You don't want to activate muscles in the hands/arms to try to make it open. The elbow (in general) is still a little bent when, it arrives in front of the back hip just before impact. The back arm will eventually straighten out after impact.
The elbows get back in front, when the body unwinding, brings them back down through impact. It is like the body "swings" the arms/club back down and through impact. I don't usually use the word "swing" because a "swing" stays on one path and is not "turning"... but it is as if it was a "swing" that the body was moving in a rotary path.
Move Less ... Get Good! DUPLESSISGOLF