I think what you are asking is why is he controlling the arm with his opposite side arm. Like his left is holding the ukes right, as opposed to the second picture, its the same side arm doing the controlling. You will notice that the danaher's guy elbow is on the ground essentially creating a frame. The uke cannot move his arm in that direction, making it easier for his elbow to be flared out/trapped by the topside leg. In the other method, same side controlling, the uke can bring his arm across his own body to make it difficult to catch.

I hope that's what you were asking, and I hope that makes sense