To clarify the above: Press A,B,X,Y at the same time while the red loading icon is on the bottom right of a chapter load. As soon as the coffin hits the ground keep tapping X while the screen shakes violently until she kicks open the coffin. Press start and go back to chapter select and repeat!
Nevermind. You don't need to press ABXY at all, just X as soon as the coffin hits the ground. The game even points this out in the "Tips and Tricks" section. It's funny to do on Chapter 4, where there will often be an enemy to hit with the coffin door. Though you can do it without an enemy being there.