PSYCHO is a modification for Quake made by Dolor_Mordax. While he didn't originally intend to upload this file, he decided to after the fun he had with it in a Deathmatch session with his friend, tsw. In particular with this mod, the player can make their teammates fly like kites around the map, granted friendly-fire is on.
This modification adds extra Gibs for all Monsters. Dolor_Mordax felt every modification being released included this and so his had to as well, though it can result in the player getting kicked out of various servers or getting packet overflow errors.
Intended mostly for Deathmatch, the Shotgun has been changed so that it isn't weak by making it the most powerful Weapon. This was done by making it fire rapidly like a machine gun while causing more damage per shot, as well as making it no longer require Shells. The Double-Barrelled Shotgun has also received such changes.
The Grenade Launcher shoots Death Knight heads that don't have as much of an arc. The Rocket Launcher is now a homing missile that moves slowly to prevent Splash Damage if nothing is seen, while moves rapidly to destroy an opponent if visible. Intended for Singleplayer, since Deathmatch should be more conservative with Ammo, the explosive Weapons don't use Rockets and can thus be fired indefinitely. Each rocket/grenade projectile causes more damage than the vanilla game. Shamblers no longer have a resistance to such explosive Weapons, instead they are extremely weak to them, meaning a Rocket Launcher can kill a Shambler in a single shot.