I'm thinking it's simply like timing a 1 frame combo in a fighting game. You just have to practice it enough to get it right most of the time, and sometimes you just get lucky, like he did this episode. Holding back won't affect the success or failure of the execution. It's either you get it right and break nothing, or get it wrong and break whatever you used for the smash. One for all lol.

To be clear, I'm not defending the concept. I find it flimsy at best because of all the mixed messages of what One for All is since the start of the show.

@Neo - Extra arms make sense to get an advantage in combat. Having the most vulnerable part of your body sticking out in an engineered fighting machine, well, is another story,