An autococker is essentially a pump marker that has been modified to pump itself. It's a closed bolt operation marker. This means that the bolt is in the forward position and is stationary at the moment the paintball is fired. (As opposed to an open bolt marker, in which the bolt starts in the open position, then upon firing pushes the paintball into position and fires.)
The firing sequence of an autococker works like this:
1. The bolt is closed and the paintball is sitting in the breech.
2. Trigger pull starts. This releases the sear, allowing the hammer to strike the valve which fires the paintball.
3. (Continue pulling trigger) The 3-way actuates the ram, which re-cocks the marker and opens the bolt so that the next paintball can drop in.
4. Release the trigger. This causes the 3-way to actuate in the opposite direction, closing the bolt and positioning the next paintball in the breech.