Engorged w blood but not erect

I’ve seen several references to a state of the penis where it is full of blood like an erect penis, yet isn’t erect (ie it much more flexible - doesn’t feel like bone inside). I’m not talking about a semi-erection, I am talking about even engorged beyond normal girth, yet not erect.

Could someone tell me how this is possible physiologically/biomechanically speaking?

I get that a lot too. I associate it with pumping results. I don’t know how to fully explain it, but I like it.