“Whole club MOI” is the physical measure of heft, the quantity that resists turning the club from cocked to released. It is a basic physical quantity that governs a golf club’s behavior.
MOI is defined by the mass and the axis the mass is rotating around. Quantitatively, the moment of inertia of each grain of mass is its mass times the square of the distance to the axis.
Any component of the club has its Moment of Inertia. The bigger MOI causes the more torque it will take to produce a certain amount of rotational acceleration. At impact the whole club rotates around the hands somewhere near the wrist hinge, so it is assumed that the club during a swing rotates around the butt.
The longer the club, the heavier the weight of the components (head, shaft, etc.), the higher the “Whole club MOI” will be. Conversely, the shorter the club, the lighter the components, then the lower the MOI will be.
Moment of Inertia is expressed in units of mass-times-length-squared. Clubmakers use kg-inches-squared or kg-cm-squared.
update: November 2017