Demo:
https://www.youtube.com/watch?v=07oOWE1UXIg
A. with rifle or pistol
1. Select rifle, do not lower it!
2. Go to crouched position
3. Move close to the wall
4. Be parallel with the wall
5. Lean to the side where the wall is (double tap Q or E)
6. Strafe to the wall direction
You will see through the wall and some cases you are able to shoot or throw grenade to the other side of the wall. It is really funny if you are in the building and shooting out.
B. without weapon
1. drop every weapon what you have
2. stand up
3. run/sprint to the wall