Is this Rainville or Swarzak?  I can get to YouTube right now, but the link has Anthony Swarzak in it...

Rainville modelled his delivery after Roger Clemens, his hero since Rainville was a kid growing up in New England and Clemens was still with Boston. Clemens is famous for short-arming the ball, looking like he is shoving the ball toward the plate. It helps with deception, as the batter doesn't see the ball coming out of the hand too well.

I don't think the slowing down mid windup is a big deal. He gets it going again in time and has good forward momentum at the release point.

The short arm protects the shoulder, but puts a lot of strain on the forearm and probably on the UCL area of the elbow too.

It will be interesting to see if he stays healthy and what his injury history is if he doesn't.