Mast moment is the bending of the main roor shaft due to the great capability of the rotor to develop pitch and roll moment (bending). In a "rigid" rotor these bending moments are two or three times more powerful than in an articulated rotor, and of course infinately more powerful than a teetering rotor, which has virtually no moment producing capability (look at the thin masts of a teetering rotor to understand how little bending it can or must transmit). Compounding the extra moment is the fact that the rotor blades flap less, and the tip path moves less, so the rigid rotor gives the pilot fewer cues as to the moment the head is imposing in the mast and on itself.
In flight, this moment produces eye-watering rotations of the fuselage (look at Chuck Aaron's displays in the Red Bull youtube videos for illustration!)
The reason for the moment gauge is that long term damage from high loads can occur on the ground. When the aircraft is restrained against the ground, the rotor moment is countered by the ground forces, so the mast can be highly stressed and the pilot has no cues. In a few minutes, much fatigue damage can occur, and the mast is a critical component, for sure. The "rigid" rotor helos also have cyclic restraint systems to help keep the mast from suffering from pilot inattention.
It is possible for the moment to be calculated from the summation of the mast moment and the forces imposed on the support legs of the static structure (or the beams cast into the main transmission housing, which serve the same function) . The question is whether the moment is a problem in the mast or in the rotor head and blades, or in both. For a specific design the moment gauge should receive its inputs from the proper place, mast and head, mast alone or head alone, whichever is the area of concern to the design team. Mostly, I think the design team is worried about the mast and its long term fatigue life, since cracks can be hidden between overhauls.
Here is a fun anecdote from back in the 1970's during the ferocious competitive flyoff between the two protptypes for the US Army contract for 1500 helicopters that became the Black Hawk:
The Boeing YUH-61A experimental prototype had a rigid rotor scaled up from the BO-105, and the Sikorsky YUH-60A had an articulated rotor (and still does). In an exchange of teasing "coloring books" that critiqued each aircraft, the test pilots let fly their comic sensibilities. Frank Duke of Boeing delivered their coloring book to the Sikorsky compass rose one day, dropped from their prototype with a small parachute. I saw it land. Inside a wooden box of bull**** and Sikorsky brochures (to establish equivilence) was a plastic bag and the coloring book, which had a number of great jibes at the Sikorsky design. One page showed a hydraulic damper standing with its back turned toward the viewer, looking over its shoulder as it pissed a red stream of 5606 against a wall. Their caption read, "I am a hydraulic damper, doing what hydraulic dampers do best, taking a leak. Color me obsolete!"
As a response, John Dixson had us make a coloring book, and John and I dropped it off a week later at Boeing's Calverton, Long Island test site. The crewperson in back was Lou Cotton (who later ran the Comanche program.) The Boeing helo needed a mast moment gauge, so we picked up on this new limit by drawing a mast moment gauge, and its needle and arc. The gauge started off on the left with a yellow scale that started out as "Uh-Oh.." and a top position that read "Oh ****!" before it then swung to the right to read "Jesus Christ!" The caption said, "This is a Mast Moment Gauge, it tells you how much trouble you are in. Color it Dangerous Red!"
Today I am sure there is a Federal Law that would declare us urban terrorists for dropping boxes from helicopters.