The best way to understand and explain moment of inertia (MOI) is to demonstrate it. I have taken a series of photos which explain how to do this. First, keep in mind that MOI is a measurement that determines the resistance of clubhead rotation on off center (off horizontal center of gravity) hits. MOI has very little effect on exact horizontal center of gravity hits.
Demonstrating MOI is easy. Give it a try!
This is a very sophisticated MOI machine made by Inertia Dynamics which is a division of Rockwell International. This is the same unit that Nike and the USGA use plus a number of other major golf club manufacturers. It is interesting to note that The GolfWorks was one of the very first companies to buy and use this machine. We have had it since 1995.
It works by mounting any object on the spindle making sure that the centerline of the spindle projects through the objects center of gravity. In this photo we are measuring moment of inertia through the horizontal center of gravity plane, thus the clubhead is mounted on the horizontal center of gravity.