Advanced Weapons Pack is a modification for Quake made by Lynx and Dumont. This modification allows for alternate Weapon modes that can be fired by pressing a button that is bound to impulse 16 if the player is carrying a specific Weapon.
Grenade Launchers can fire Proximity Mines (costs 6 Rockets), which are similar to grenades but only explode when an opponent gets close; the player will not trigger their own Proximity Mine. Rocket Launchers can fire Homing Missiles (costs 5 Rockets) that can track and be drawn to opponents.
Impulse 20 can be used to spawn a Backpack with some of the giver's Ammo, up to a maximum of 20 for all Ammo types besides Rockets, which only have 10. Impulse 26 will detonate a Grenade (not a Proximity Mine) a second or so after being pushed.
Note that this modification has a glitch whereby a player can continue to fire the alternative mode of a Weapon, even after they run out of Ammo. As Quake doesn't allow the player to switch to Weapons they have no Ammo for, this glitch can only be exploited if the player continues to use the same Weapon.