The gas pushes the
Bolt CARRIER backwards -- but before the carrier moves, the gas flows in to the rear cavity and expands, pushing the bolt forward very slightly for a fraction of a second to unload the jugs and ease unlocking and rotation (caused by the beginning rearward movement of the bolt carrier camming the bolt counterclockwise). LINK.
The three little rings on the bolt (inside the red goose egg on the picture below...) are effectively piston rings to enable that.
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