[count-your-change]

I did watch video and then went to the engine home page. There are apparently some parts not shown in the drawings nor fully explained.

It sounds like there is a small piston or set of seals that does not leave the cylinder and the larger piston simply pushes against it.

[TigersEye]

I don't think there is any piston that stays in the cylinder. The seal is created by the compression ring cartridge in the the compression ring assembly. The rings apparently are the only thing that touches the piston and nothing touches the cylinder walls.