Every weapon in Perfect Dark (with the exception of the "Classic Weapons") features a secondary function that often drastically changes how the weapon is used. Secondary modes vary from simply using pistols as melee weapons in close combat to turning assault rifles into explosive booby-trap devices, as well as many other elaborate functions.
Secondary functions in Perfect Dark are activated by holding down the B button on the N64 controller. The majority of secondary functions are toggled on or off and will continue to take effect until switched back to the standard function, although a few must be activated (with the Z button) while the secondary function command is being held. Secondary modes are designated by a yellow square in the bottom right corner of the screen, while standard functions are shown by a red square; text explaining the different modes will also briefly appear upon activation.
Most secondary functions are unique to a particular firearm, although a few are shared across several weapons, most notably the Threat Detector and pistol whip modes. Even unarmed combat has an alternate function, which dramatically reduces the damage dealt with fists but immediately disarms enemies of whatever weapon they may be carrying as a trade-off.
The concept of giving weapons alternate firing modes was generally unheard of when Perfect Dark was released, and while the concept is commonplace in modern first-person shooters, it is rarely applied to more than a handful of in-game weapons, as opposed to most of the weapons in Perfect Dark.
Perfect Dark Zero
Perfect Dark Zero also features secondary functions and even tertiary functions on some weapons, and they are activated through a combination of buttons (usually Right Bumper).