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I hold my pick fairly loosely, and I place the pad (not the tip) of my thumb on the pick. Picking down, my thumb is a "backstop" to the pick.
But I notice that exactly where my thumb rests on the pick affects the solidness of the tone as well as the amount that the pick "flops" when I pick down (for soloing).
Moving my thumb a millimeter or so, or letting it slightly bend back, greatly affects the feel of playing and the tone. Maybe it's personal to each player's anatomy and preferences, but I wonder if there's a consensus on exactly where and how the thumb rests on the pick (assuming no thumb indent). The more the pick flops (with the grip not tightening), the more it feels like the wrist is plucking the note. (Not sure if this makes sense to most people!)
I've seen videos of at least one well-known player where the pick flops back and forth. Maybe each person has to find his or her own way.