There's a certain spot if you move too far forward, your head will be sticking out. You'll learn from experience where that spot is, but if you're not forward enough your shots won't go through the rock.
Note: Your legs are exposed from behind, but unless someone is actually on the rock then you can't be killed, also from the kill cam it looks like you're ontop of the rock so when they spawn and check the rock it will look like nobody is on it.
Enjoy.