The arm servo is an upgrade found in singleplayer only a total of three times. The player is given one for free after the level "The Sniper's Lair", where in a cutscene Glitch re-assembles the droid Spy disguised as a Mil, known as Agent Goff. Goff gives the player an Arm Servo upgrade, the other two can be bought from the Barter Droids.
Glitch's Arm Servo determines the speed of his arms. As he gains Arm Servo upgrades, he can switch weapons faster and even reload most weapons faster.