Tekkai, aka the "iron body" technique, where the user flexes all of their muscles at once, making their body hard enough to defend against almost any attack...but there are no major muscle groups in the boobs...
It was rendered pretty much irrelevant by armament haki later on in the series...frankly I think inventing armament haki was one of Oda's mistakes but hey, I didn't write the world's most successful manga series so what do I know?
Tekkai, aka the "iron body" technique, where the user flexes all of their muscles at once, making th
