Gear Effect
The spin induced on the ball from off-center hits due to clubface rotation.
Full Definition
Gear effect occurs when the ball is struck away from the center of gravity of the clubhead. The clubface rotates around its center of gravity at impact, imparting sidespin on the ball in the opposite direction of the miss. A toe hit causes the face to rotate open, but gear effect adds draw spin. A heel hit causes hook spin from gear effect despite the face closing.
Why It Matters
Understanding gear effect helps explain why your shots curve. It's why a toe hit on a driver often draws back toward target, while the same miss with an iron (which has less gear effect due to its smaller head) produces a push-fade.