Gyroscopic Precession
Also: Precession
The way a fast-spinning bullet answers a turning force by circling its nose around rather than tipping directly, letting it nose over along the trajectory.
A bullet leaving the muzzle is a small gyroscope spinning tens of thousands of times per second. When the airflow tries to push its nose off the line of flight, the spin makes the nose respond ninety degrees away from where the force is applied, so instead of tipping over it walks in a slow circle. This is gyroscopic precession, and it lets a sufficiently spun bullet track the curving trajectory and keep its point forward as gravity pulls the path down.
Whether precession keeps the bullet stable or lets it tumble depends on the stability factor, which in turn comes from the twist rate and muzzle velocity. The same circling response that holds the nose into the wind also produces a small steady lean called the yaw of repose, and that lean is what creates spin drift over distance. Precession is therefore both the reason a bullet flies point-first and the seed of a predictable lateral error.