Attackers 1 and 2 are on the blocks, defender X3 stands between them facing coach, who passes to 1 (or 2) and X3 "walls up" aggressively, moving his feet with arms extended (up), staying attached to the ballhandler and not fouling.
The basic technique of walling-up is to keep your body and hands straight up. Walling-up occurs most often in the paint to contest shots and avoid fouls. The hardest thing to teach is to keep the feet and lower body moving to take up space before the ballhandler stops his dribble or pivot. Wall up on the perimeter to take away vision of a shot or pass.