Cup the wrist to keep the lob face open at the top
A common fault is the clubface getting shut in the backswing. The instructor wants the trail wrist cupped to about a 65-degree maximum so the face stays super open at the top, and notes that on this shot you can't overdo the open position, only underdo it. He suggests a simple home drill: set your hand into that cupped position and hold the club there.
Channel: Dan Grieve
Watch official source at 5:05