'Mechs constructed without hand and lower arm actuators in both arms can flip their arms over and fire backward. This ability only applies to 'Mechs originally constructed without those actuators (or OmniMechs that remove their hand and lower-arm actuators for a certain configuration), not 'Mechs that lose them due to critical damage.
A 'Mech that intends to reverse its arm-mounted weapons must flip both arms during weapon attack declaration. This maneuver takes the place of a torso twist (a 'Mech cannot torso twist and reverse its arms simultaneously). After flipping its arms, the 'Mech may then fire any arm-mounted weapon into the rear firing arc instead of the usual firing arcs for those weapons. The one exception to this rule occurs if weapons are split between the arm and torso. While the arms can still flip to fire weapons that do not have a split location, split-location weapons can only fire into the front arc.
If a 'Mech that can reverse its arms loses one during a game, it can still reverse the remaining arm. During the End Phase, reversed arms automatically return to the standard front arc.

Prone 'Mechs and ProtoMechs: Neither of these units may reverse their arms.