Keep the lead arm straight to hold your swing circle
Think of the golf swing as a circle drawn like a compass. If the left arm stays straight going back and coming through, the club returns to the same spot every time. When the arms shorten, the circle narrows, and you end up hitting above or on top of the ball.
Channel: SagutoGolf
Watch official source at 1:40Why this context matters
Coach's framing for why topped shots happen.