With the attachment equipped, you could either shoot normally or press the left button to change to the Gangsta/Bandit Shooting Style. Your vertical recoil would become horizontal recoil, which would make guns like the Skorpion EVO superb for clearing rooms and/or inducing rage.
You would still be able to ADS, but your gun's recoil would be greatly increased (something like, say, -2% centerspeed).
Oh, and this attachment would be for all weapons. Even the Assault Shield would benefit (like for those pieces of cover that already cover up your legs; when behind those bits of cover, the lower part of the Assault Shield becomes completely useless).
This is the part of the topic where I would refer you, the reader, to a Youtube video of some Call of Duty 4: Modern Warfare: Reflex Edition Wii gameplay footage showing off some Gangsta Style Shooting. Unfortunately, it appears nobody bothered to capture gameplay footage of Gangsta Style Shooting for Call of Duty 4: Modern Warfare: Reflex Edition for the Wii. So instead, I hereby present to you some random chump's 7 view video that is somewhat relevant: http://www.youtube.com/watch?v=ovxZz-nVUnI