Body movements can alter one's center of gravity significantly. Just
as a high jumper can clear a bar without ever having his center of
gravity above it, the child on the swing can move his legs in or out,
and bend at the waist, to move his center of gravity just a touch
higher on each swing than the height attributable to the energy of
the previous reciprocal swing. On the downstroke, he repositions his
body again to prepare for the maneuver on the other side, which uses
the energy of his muscles without depleting any of the gravitational
potential energy he has gained. The amount of extra energy put into
the system from his own muscles is enough to escalate each swing.