The primary function of the Mauler works exactly like the Super Shotgun from Doom II, consuming 20 energy cells to use. Human enemies are disintegrated when killed by the Mauler. This is the same firing mode Templars use. The Manual calls this the 'Scatter' mode. Small particles could be seen behind the green "screen", when weapon is in this mode.
The secondary function (activated by selecting the weapon again) uses 30 energy cells, but shoots a large ball of energy which can kill almost anything in one hit, sometimes even Inquisitors in a single hit by chance or if the Inquisitor is hit dead center. The player will lose 20 health when using the secondary function. After the projectile hits something, a circle of smaller plasma balls will come out to hit other enemies. The Manual calls this 'Torpedo' Mode, and it indicated by big ball behind the green "screen" on weapon sprite.
It is advisable to avoid firing in the secondary mode in close combat. The initial blast can inflict significant damage back on the player. The secondary projectiles do not cause any damage, however.
The Mauler only appears in three levels in the whole game, the Fortress Warehouse, Factory: Conversion Chapel (inside a secret room) and the Proving Grounds (also inside a secret room).