The Swing is at the end of a 50-foot rope.
I find the best technique is to grab the swinger by the feet and pull 'em up (as Joseph is doing here), let 'em go and then push 'em on the rebound.
Also fun is to grab the rope's end sticking out of that knot and turn in a tight circle, causing the swinger to go 'round instead of in an arc.
In both cases the trick is to avoid the swinger contacting the tree trunk :)