"Laser-guided lightning" is how Cerberus operatives sum up this unusual weapon developed for use against synthetic enemies. It was created for field operatives who required an anti-geth weapon but found omni-tools too complex to master or too hard to fire in the throes of an adrenaline rush.
To make it as simple as possible to shoot, the arc projector has a single trigger that takes the weapon through three operations. Depressing the first stage of the trigger causes the weapon to irradiate a target with a neutron-driven gamma-ray laser, which projects a beam invisible to the naked eye. This beam ionizes a thin corridor of air and any medium it passes through, including most solid targets. When the second stage of the trigger is pulled, the projector generates a massive electrical impulse. This arcs down the ionized corridor, causing trauma to living targets through tissue burns and mechanical force (convulsions). Synthetic targets' circuits typically melt and fuse. By 22nd-century standards, the weapon's power needs are not large -- it kills less through absolute amperage and more through sustaining the current for more microseconds than, say, an actual bolt of lightning.
It is during the third stage of the trigger pull that the projector's autotargeting system works to its full capacity. While it is still being projected during the electrical attack, the laser is reflected by movable optical fibers in the weapon's business end to "paint" additional targets designated by the gun's targeting computer. Given a new path of least resistance and surrounded by targets of differing potential, the electrical jolt then arcs from its first target to hit additional enemies. The effect on massed mechs is predictably devastating, as are the psychological effects on living foes, who suddenly discover that neither shields nor cover can keep them safe.