I'm not clear enough on my thoughts yet to ask the formal rules question people, so I wanted to get a sense here if my reasoning is correct.
1.) Incoming damage locations are based on the facing of the mech's feet.
2.) Twisting a mech's torso changes that mech's firing arcs, but does not change the hit locations of incoming damage.
3.) A shield, actively used, protects the arm, the side torso, the center torso, and the head. It does not protect the opposite arm, or any rear location.
4.) Tripod mechs may torso twist 360*
5.) Weapons in locations protected by an actively used shield may not fire.
Therefore:
A Tripod, facing north, with a shield actively used in each arm, and -rear- mounted weapons, could torso twist to bring its rear arc to bear, fire them freely to an opponent to its north, but return fire would be resolved against the -front- of the Tripod, and have to punch through the shield.
Obviously, IF the above is true, it would get house ruled at my table pretty quickly, but as I was fiddling with 'making things hard to kill', it came up in my head.