I'd be very interested to hear what @Than_Bogan would have to say about this thread. It's been sooooo long since I've studied physics / dynamics but my guess is that it is not so much about shortening the string as it is about adding energy to the system. By pulling down on the string the Prof is adding a new force which is causing an effective increase in the tension of the string which causes an additional acceleration. I'm with @skibrain re his first post in this thread, I think the key takeaway for me is that the timing of adding this force is key, i.e. load late and get off it late. For those of us who are impatient in the turns, this is something to really think about