[Etzwane]

I could not relate "effective mass" to a precise physics concept. As you said mass plays a role in kinetic energy (multiplied by velocity squared) but it also plays a role in momentum (mass x velocity) and acceleration (proportional to the force applied and inversely proportional to the mass). One common aspect of conservation of momentum when describing the feel of a golf swing is inertia.

In terms of feel in a golf swing, my guess is that these physics quantities are mixed: lag pressure and longer club makes you feel inertia (momentum or angular momentum conservation), increased lag pressure is about acceleration, a longer club will resist more acceleration but it you achieve the same rotation speed it will give more club head speed. More effective mass would need a more deliberate and steady action but a better resistance to deceleration through.

As to how "effective mass" related to the physics of impact, I have no idea.