The guy giving the seminar can't be under the other guy, if his own foot pops up as a result of the push. Now with that in mind how does that effect the rest of what he is demonstrating?
Did you mean the pusher got under the seminar instructor? I'm not convinced that is the case, rahter I think he simply gave a push until it overcame what the pushee could take.
I think the pusher got over or
under the pushee's linear ground path and
overcame what pushee could take because the pushee did not absorb and re-direct the energy of the push.