I can only speak about how I throw, some things may be different with others. But it looks to me like you're trying to push the ball. Relax your arm, let gravity work for you and that should help keep your arm straight through release. Don't tense up or grab the ball. You should do the exact opposite. If you want more back-end on the ball to drive through the pocket, lift the ball off your fingertips while throwing it out over the lane.
Hope this helps. I know one thing for sure, if you don't want to bend your elbow and you're aiming for a natural arm swing, just relax your arm and let it act as a pendulum.
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