It's quite simple-there is no contra piston, merely a fixed head with a combustion chamber (or even a blind bored cylinder)-the crankshaft is carried in a bush which is eccentric-in other words-the OD and the ID of this bush are not on the same central axis-but offset. This bush can rotate within the crankcase............so if a lever is attached to the bush, the bush can be turned through perhaps a 45 degree arc of movement. This movement-because of the eccentricity-will cause the crankshaft axis to rise (or fall) as a consequence.....since the piston and rod are also attached, it follows that the piston will travel higher (or lower) in the cylinder as well-since the head is fixed, the piston will come closer or further from the head depending on where the lever is set. Simple-we have a way of adjusting the compression ratio.....
The limits are set both by the orientation of the bush within the crankcase-and also by the limits to the lever movement-in some engines it operated in a slot in the casting itself-so the length of the slot limited how much lever movement there was, or in a cutout sector of the crankcase nose-again the length of the arc cutout limited lever movement.

I'll try and put together a diagram and post it later............

ChrisM
'ffkiwi'

Edit: diagram added