Go from a run to a roll without stopping by pressing "X". That's for the PC version, but I think I saw that someone said it works for the PS version as well (although I wouldn't personally know if there is an X on the PS controller).
I got stuck here, myself, and I know others have too. The run-to-roll function seems to be poorly documented, even though essential for getting through the game.
Good luck and have fun!
