Don't you love the mechanics of this first vid? If you push directly along a line whereupon both feet are planted, that will be against a strong stance. If you direct your force downward -- as is obviously being done here -- that just roots the stance even more.
If you change the push so that it is upward, and slightly angled against the line, and apply more force, as is being done after the student's arm is lowered, you take the weight off the lead foot, and against somebody holding themselves rigid, it's relatively easy to move them, even if they don't help.
Whether the student being demonstrated upon raises or lowers his arm has nothing to do with anything, least as far as physics and biomechanics are concerned, this is to say, zip, zero, nil, nada, bupkis.
Unless, of course, he is holding onto Wonder Woman's invisible ship's hawser when his arm is outstretched.
You have to be careful putting this stuff up where people can see what is really going on.