I think m_1 is roughly your butt, and m_3 is roughly your shoulders. Phi is the angle your body makes wrt the chain. So pulling on the chain changes phi. I thi

I think m_1 is roughly your butt, and m_3 is roughly your shoulders. Phi is the angle your body makes wrt the chain. So pulling on the chain changes phi. I think the idea is that three masses can model a center of mass that may not coincide with the butt as well as moment of inertia — mass, moment of inertia and CM displacement is three parameters, and three point masses gives enough degrees of freedom. But three point masses cannot model non-rigid-body effects. The offset of CM from butt can vary (by bending one’s knees if nothing else), the CM offset from the swing can vary in two dimensions. I don’t know how much this matters.