Darner, I was/am not pointing out you or anyone in general. A lot of people on forums doubt or ignore NextLevel when he is in fact handing out free and very practical effective advice. I take many opportunities to support him in my warped way.
Short answer I say is I loop pretty much the way you describe, explode up from a crouch impacting the ball when I am almost all the way up to my normal ready position stance, not exactly standing straight up like a tent pole, but almost all the way up to my playing crouch, which is a little higher than most. I think it is all about using every part of your body to create energy and funnel it into an effective shot. The whole body offers power and control.
As for myself and my way to loop underspin balls, I certain do use the whole body. I crouch with extra knee bend and make sure my entire wist is below table, I drop my arm/wrist down between my legs and cock my wrist back nearly as much as it can go, I wait for ball to come into my zone, I explode upwards and let it rip, I go for either a fine brush for spinny slow loop, or a fast loop if the ball is a bit high or opponent is camped at one end. Like Carl mention, the wrist snaps almost on its own in hte right time if you were loose and well timed.
NextLevel always emphasizes the elbow position. I always emphasize that you do not keep arm locked and try to pivot on shoulder joint. Like NL, I believe upper arm comes up with the body, but it stops at a point. This is what I call your leverage. All that kinetic energy is flowing from your leg and hip explosion, you simply let the arm (both upper and lower) move, but at a point a little above where you impact teh ball and a bit to the front of your body, your elbow stops and parks itself while the lower arm continues up, and you accelerate the wrist right before impact. This maximizes all the nicce energy you produced and channels it to the ball.
Whether you choose to do a slow spinny loop or a fast loop depends on your judgement. The slow loop is exploding upwards, the fast loop explodes up and forward.
I think BH loop is a real natural shot and the loop vs underspin in my mind has got to be the easiest loop to do, even if the FH is way more dynamic and possible to do on the move, there is something very satisfying about making a really spinny or fast loop against an underspin ball.