They call it the "clutch". Up until now, fervent scope-users have been pressing thumb-accessed "sniper" buttons to activate a mode that lowers DPI (dots per inch) thereby allowing them to shoot the fleas off a dog's back at 500 yards. The Basilisk does things a touch differently with a removable (and therefore customisable) paddle that juts out a little from the main unit. It looks a tad ugly and more than a little flimsy, to be perfectly honest with you, but what it lacks in aesthetics it more than makes up for with head-popping results.
Basically, I can't go back to regular old thumb-button sniping now, and I also refuse to put up with any mouse that lacks the Basilisk's other major feature, scroll wheel resistance adjustment. Using a roller neatly hidden underneath the unit, you can change the ride feel of the scroll to suit your taste. I'd describe the two spectrum extremes as "oiled wheel of a Matchbox car" or "rusty old wheel of fortune prize spinner". Going in, I never thought I'd care, but that was yesterday – being allowed to find my preference on that spectrum is a plus that puts this mouse slightly ahead of the pack in my opinion. Bottom line: FPS players who are unafraid of trying unconventional paddles ought to give the Basilisk a look.
Along with the above mentioned, Razer's comprehensive Synapse 3.0 software allows you to setup keyboard functions, dig into a fully fledged macro editor profile switch, and mess with the RGB lights. You can also make the clutch paddle become a “hyper shift” toggle which changes the function of every button on the unit (essentially doubling the number of buttons you have). Make smart use of the feature and you could have 16 functions per profile at your fingertips, which is great news for MMO players.