Named after German chemist and physicist Heinrich Gustav Magnus who experimentally investigated this in 1853.

Physical phenomenon in fluid dynamics and continuum mechanics. The Magnus effect results in a sidewise force on a spinning sphere or cylinder, when the sphere or cylinder is surrounded by a fluid (gas or liquid) There also has to be a relative motion between the fluid and the sphere/cylinder body. The force, per unit length of the spindle, is 

F = ρv0K

where ρ is the density of the fluid,  v0 is the perpendicular velocity of the fluid far away from the cylinder and  K is the circulation around the cylinder. This is directly analogous to the force that a transverse magnetic field B0 exerts upon a wire with an electric current I, which is per unit length of the wire equal to B0I . 

This effect - among other things - determines the parable of flight for a golf ball, baseball or a tennis ball that's hit. It also affects the trajectory of  fired artillery shells, since they are spinning too.

The way it works is that because of the moving - and spinning - body, the fluid around it experience differences in the velocity in different parts of it. This results in pressure differences, which in turn make the moving body depart from its straight path. The Magnus effect is all in correlation with Bernoulli's theorem (Bernoulli effect) in fluid dynamics, but with the object spinning instead of still, as Bernoulli assumed. The key of it all is the interaction between the body and the fluid in the boundary layer, which was theoretically introduced in 1904.


reference:  D.J. Tritton; Physical Fluid Dynamics