The ear of the bolt is atop the wide part of the hammer's cam with the hammer down onto the cylinder's cap/nipple, or the SAA's hopefully empty chamber if you haven't fired yet.
When you cock the revolver, the cam carries the rear of the bolt upward, releasing the bolt head downwards from the cylinder notch just before the hand engages the rachet, allowing the hand to turn the cylinder. At the point where the bolt's extension has been adjusted to slip off the cam, it drops, raising the bolt into the (next) cylinder notch at or about full cock. Pulling the trigger rotates the hammer forward, and the ramp on the cam pushes the bolt's extension sideways to reset atop the cam's wide surface, ready for the next shot.
Recocking the hammer starts the cycle over...