In the burst position, there is a cam that rotates with every shot discharged. Once that cam has rotated it's number of shots, generally 2 or 3 rounds, it allows the disconnector to engage the hammer. Releasing the trigger causes the disconnector to let go of the hammer and allows the trigger sear to hold the hammer in the cocked position. The next trigger pull starts the sequence over again. In full auto, the disconnecter doesn't engage the hammer. In semi auto, the disconnector engages the hammer after every shot, requiring the trigger to be released before it can fire again.
