Soft rope ( the platted type ) down the spark plug hole till it get jambed between the piston & head
A long bolt down the timimg hole to do the same thing
holding the crankshaft nut at the timimg side
putting it in gear & locking the brake or rolling the front wheel into a wall so the bike can't move
A socket place in the clutch chainwheel to jamb the primary chain
probably a fewmore that did not come to memory
All of them have potential problems so must be used with care
The rope down the plug hole is the one least likley to do damage but hard on a side banger where the plug is not over the cylinder .
I have a 6 spring clutch in my bike, so that is not as much of an issue.
My cush drive tool is similar to the one in Ron's second picture, but requires a cross shaft to operate. Leaving aside holding the engine from turning, it is clumsy to operate as one hand is needed to hold it in place and two more hands to apply force with a cross shaft.
Thinking I may be best served to modify my tool to add a means of using a power tool to operate it.
this is a recurring issue, maybe time to put it in the technical section. :thinking_face:
I use a simple bar of aluminuim placed as in the picture (turn around to tighten/ loosen the cush nut). No need to lock the rear brake or stuff string in the cylinder.