The pistons are all rotating, so there is compression coming from the piston behind.
There are 8 ... 4 sets of 2, yet individual ...
piston 1 pushes piston 2, then they trade off (piston 2 acts as piston 1 against the next piston) and so on.
My guess is .. Piston 1 hits piston 2 and rebounds to an exhaust port where the residual pesssure is relieved. While there are no valves, there are ports that open and close.
I'm guessing there are holes on opposite "sides" of every other piston.
I'm sure there will be more info about this engine as time goes on. But, for now, we can guess.