The main problem with the rope trick is that, depending how much rope you feed in, the piston might be close to top or bottom dead center when you apply torque to the bolt. If this happens it is using the crank/rod/piston as a force magnifying system to increase the force on the piston crown, potentially to astronomical levels.
If the amount of rope is just right, the piston should be right at mid-stroke when you apply the load, and the force on its crown is only the force you put on the breaker bar multiplied by the length of the bar and divided by half of the engine's stroke - that is, the piston crown will probably only see around ten or twenty times the force you put on the breaker bar. However if you don't feed in so much rope, and the piston is near top dead center, the force on the piston crown will be hundreds, or even thousands, of times the force you put on the breaker bar. This is much, much greater than the force it takes to crush the piston crown.
In other words, it might be OK if you understand the mechanical principle and therefore feed in the right amount of rope - but if you don't it is quite likely to crush your piston, and may even break your connecting rod as well.
