The poppet seals the compression chamber is sealed by the poppet.  The poppet is actuated by the poppet stemp being forced down by the hammer.  The poppet contains a return spring that pushes the poppet into the transfer port opening to seal in the compressed air.  Once the pressure in the compression chamber increases both the return spring pressure and the air pressure hold the poppet closed.  Daisy uses a crimp clip to retain the poppet stem.  To disassemble the poppet the crimp clip must be removed.  The video in this post illustrates how to remove the crimp clip. Replacing the spring with a lower rate spring will decrease lock time and reduce valve lock. Also replacing the crimp clip with washers and an e-clip will allow tuning the poppet lift, hammer travel distance, and increase the contact surface area of the hammer.