The slowing down of the elbow is just before you strike the ball, to increase the rackethead speed. Just before you strike the ball, as in your whole arm is moving forwards and up, then just as the racket gets to horizontal, a millesecond before impact, the elbow slows down...it's done it's work already, and you slow it down to WHIP, like a bull whip, to get the rackethead, not the rackethandle, to move fast.
I think one of the first responders mentioned how far into the court you hit the ball. I agree, you seem to toss too far into the court so you can't use the pronation/whip effect at all. It's OK to toss that far into the court, IF your upper body is also that far into the court.