@greatninja3
that’s why he did not want to use it cause being sneaky is not his personality
Also, he didn't have much chance to use it. The first time we saw him in action, his opponent gave up immediately. The limitation was only explained as 2 seconds invincibility, but could he only activate it for a fraction of a second, like say, to receive a hit, and then save the remaining time for another hit? If he only activates it whenever he's about to take a hit, then he would effectively be able to defend against 8 attacks considering average human visual reaction time.
We could speculate forever about how the ability could be effectively used because the manga hasn't explored it yet but that still doesn't answer the question: is he truly invincible? can the cutting ability really
cut through anything?
This is like that shield and spear merchant dilemma.
https://en.wikipedia.org/wiki/Irresistible_force_paradox
And yeah, we still don't know much about the old man's ability. I had assumed that it was also like a shield type but now that I think about it, it's probably more like it just stops/pushes away anything within a certain distance from his body, and if he extends the range, he gets a severe recoil.