20:20, and 23:30(where he is holding the guy back one handed) doesn't look anywhere like a locked arm to me. If he was to lock it from that position, you would see a twist in the bigger guy each time he pushed.
At 20:20 (http://www.youtube.com/watch?v=FoMoPNz4W8M&t=20m20s
his arm is still under the armpit of the strongman.)
At 23:30 still there as well (http://www.youtube.com/watch?v=FoMoPNz4W8M&t=23m30s
You guys should be looking for the back arm, the one that is away from the camera. It is locked under the guys armpit. It never moves. If there is a video where this is not happening, I would love to see it.