Bladder-style caps feature a rubber bladder at the top of the shock. As the shaft enters the shock, the additional volume of the shock shaft compresses the oil and puts pressure on the rubber bladder, which compresses the air trapped between it and the cap, generating rebound force. Bladder-style shocks are intended not to have air inside the oil-filled part of the shock.
See our shock caps article for details on how bladders affect handling.