The Magnus Effect occurs when a spinning object moves through a fluid. The spin drags the boundary layer, creating faster flow on one side and slower flow on the other. By Bernoulli's principle, faster flow means lower pressure — so the ball curves toward the low-pressure side.
The Magnus force is F = CL ρ ω r² π v, where ρ is air density, ω is angular velocity, r is radius, and v is speed. Topspin (positive rpm) curves the ball downward; backspin curves it upward. This is why a curveball curves, a sliced golf ball veers, and a topspin tennis shot dips sharply.