Piston stop

From MopedWiki

A piston stop is a tool that screws into the sparkplug hole in the cylinder head, and extends downward into the cylinder. This tool stops the piston from moving, which in turn makes it much easier to work on engine components connected to the crankshaft, such as the flywheel and clutch drum.

Don't have a piston stop or don't want to drop the cash on one? Try the rope trick!