Two kinds of spin: topspin is brushed low to high, flies in a high arc and dives steeply. Slice is undercut — a flat flight path and a low, skidding bounce.
Spin is the safety margin that lets you hit harder without missing more. Topspin comes from brushing up the back of the ball: it clears the net high and still drops steeply into the court — which is exactly why the semi-western grip became the modern standard on the forehand. Slice is the opposite. You cut underneath the ball, it travels flat and stays low off the bounce, stripping your opponent of pace and rhythm. The most common fault is forcing the spin from the wrist: that produces mishits and loads the forearm. Spin comes from the path of the racket, not from a flick of the hand. And one myth refuses to die: slice is not a fallback for players who cannot hit topspin, it is a deliberate change of pace — and on a second serve that arc is what keeps your double-fault count down.