i think in general its a script that empties the berretta as fast as possible... its generally found to not be effective because you cant move while shooting (executing a script). The only one ive really heard people using was mousewheel end even then i find it easier to click as well.
In general there are a number of bugs or exploits inherent to the game code that are generally frowned upon by league play and such made "illegal." They are generally punishable by match forfeits and possibly a short suspension but not considered hacking. Use of them in pub games is "ok"
Examples:
1) Unclipped areas on maps. In general the rule in FTW is that if you can't get there by yourself in a single jump it's off limits. An example is the roofs in prague or the top of the one building on uptown.
2) Sr8 drop: Apparently if you quickly drop the Sr8 after firing it will switch to your pistol/secondary without going through the reload motion. This is considered illegal in league play.
---- As a side note: I personally think the Sr8 reload should be similar to the SPAS in hitting the reload key after each shot. In "real life" if and enemy is coming toward you and you miss a shot your not going to spend time fiddling with the bolt action sniper rifle, your gonna toss that thing and pull out another weapon
Not sure one the retta script in FTW but I'm pretty sure nobody uses one