That might mean it has a punch I effect 50% of the time. Not sure how it would work with punch on a bow, but I do know that's how it works with kb reisst, because I know for a fact that with a kb resist .8 item you take 100% of the knockback 20% of the time, with the other 80% you don't take any.That is what punch 0.5 is for :p
Locked as the owner requested it