The gyroscopic effect

The overturning moment M_W tends to rotate the bullet about an axis, which goes through the CG and which is perpendicular to the plane of drag, the plane, formed by the velocity vector v and the longitudinal axis of the bullet. In the absence of spin, the yaw angle d would grow and the bullet would tumble.

If the bullet has sufficient spin, saying if it rotates fast enough about its axis of form, the gyroscopic effect takes place: the bullet´s longitudinal axis moves into the direction of the overturning moment, perpendicular to the plane of drag. This axis shift however alters the plane of drag, which then rotates about the velocity vector. This movement is called precession or slow mode oscillation.