A barely warm, damp scooper will melt the ice cream slightly and allow you to really dig in with the scooper. The water acts as a lubricant. So instead of hacking away at your favorite frozen treat and producing thin, anemic chunks, this hot water method allows you to glide through the ice cream, resulting in fat, luscious scoops. She would also make several short passes with the scoop in the same area to really build up the ball of ice cream (as opposed to long, shallow strokes).
It's important to rinse off after each scoop and be sure to give it a shake to get rid of excess water. You don't want too much clinging to the scoop.
Happy scooping! And thanks, Mom!
Related: The 5 Best Ice Cream Scoops