Spin drift is caused by the spin the of a bullet or projectile. The surface of a spinning bullet spins the air around it. Because the air can not continuously spin around the bullet in a constant motion a low pressure point evolves in the direction of the spin. The bullet by way of higher pressure must move in the direction of the low pressure. If a bullet or shell has right hand rifling then it will drift to the right. The same is true for left hand rifling.