This has been a constant source of annoyance for me. Scattered around the castles are darker walls where projectiles and - occasionally - players can squeeze through. Often there is the explicit expectation that you should be able to squeeze through such an opening, but nothing I've done so far seems to do the trick.
A specific room in the Forest that would have you go up and then down at the other side has such an area at the bottom that you can dash through. This seems to work nowhere else, however.
So: how do you do that? Is it the flexibility perk? Is it some kind of NG+-only rune? Is it some weird input combination? Is it something awfully obvious?