Not sure if this is the correct forum for miniature rules questions, but:
According to Page 11 on the Miniature Rules. In the example it shows a mech in partial cover, in the example the legs and left arm are covered for purposes of partial cover. This prevents both fire striking the target's legs and left arm, and the target returning fire with those same limbs. However if the mech rotates, it can bring it's left arm into arc allowing those weapons to fire.
Now, logically, if the mech rotates to bring its left arm to fire, that same arm would now be subject to return fire (and lose its partial cover). Is this indeed the case? It's not explicitly stated either way. Personally I would assume, that if a location can fire from behind cover at a unit. That same unit can hit the location being fired. Because it makes no sense that an arm for example can shoot and yet not be shot at.