Hold a pistol in your hands, lay down with a wall on your left, and aim in with R2. Tap R1 to roll to the left and you'll see you start breaking through a bit. Tapping circle mid-roll will get your character stuck halfway in the wall, and you can move about 120 degrees more before being pushed back out. You must keep holding R2 and left analog. Video shows the use of this for PVP, as well as looking through walls. Works on all objects except for cars with intact glass. To go through a car you have to break the glass.

Video Example