The spin starts by slackening the grip on the drumstick so the drumstick is held between the thumb and index finger. A small twist of the wrist is used to start the stick spinning. Just as the stick moves past the point where the thumb grips it, the middle finger should move to the other side of the stick from the index finger. At this point, the stick is spinning, being held by the index and middle fingers. As the stick moves past where the index finger moves, the ring finger should move to the opposite side of the stick as the middle finger. Some people will continue and use the pinky to finish the spin. This is more difficult and tends to not be as visually pleasing as a tighter-looking three-finger spin.
The spin is usually stopped at this point by grabbing the stick. The spin can be continued indefinitely by resetting the stick to the starting position of the spin. The motion of the stick is used to move it around the opposite side of the fingers from the spin. The stick will briefly be held by the index finger and either the ring finger or pinky before it continues the spin between the index and middle finger. Make sure the spin reset looks just like any other part of the spin. Since the stick is going around the opposite side of the hand, this can initially be hard to achieve visually. With practice, a drummer can spin the stick indefinitely without the audience being able to tell they are cycling through a short spin move over and over again.
The drumstick is only held between two fingers during the spin, which will usually cause it to be uncontrolled to use directly. The spin is a good trick to use when one hand is not needed to be used during a particular part. The ease of being able to continuously use the spin is an advantage for these types of breaks from using one of your hands. Always keep in mind that there is a slight pause before and after the spin, so be sure to finish it will plenty of time to get a proper grip on the drumstick.