Well you still have to follow the basics, despite holding back. Guard up high, throw proper punches, stand properly, move properly, etc.
I accidentally knocked out a young kid a few years ago. He threw a lazy jab and I warned him that it was lazy a few times. After a while I decided to parry it and throw a very held back right hand over the top. Unfortunately he walked straight into it and collapsed like a sack of shit. I was only just able to catch him. Kid went mad when he regained consciousness but he understood my point when he calmed down
I also put a girl on the canvas with an ultra weak liver punch. Both were accidents, I swear