This guy Endospink has a lot of wierd videos on YOutube and I remembered this one and retrieved it. He shows how you mount a tall horse with a swing up. Somewhere there's a better video explainging the physics of the technique, but the really important thing, and you can see it in the video, is the when you thro your right leg up, that you don't also raise your head up. In fact, your head must actualy swing downward to counterbalance your upward swinging leg and not deaden the momentum.
Watch the first guy who swings on and you will see his head actually dip BELOW his center for a split second. The other guy who tries and fails does not allow his head to swing downward and thus he cannot swing on.