A mass (the swing) hangs from a rope that passes over a pulley and is connected to the support post. A second shorter rope hangs freely from the horizontal section of the main rope.
Start the pendulum mass oscillating with a small amplitude. When the pendulum gets to its lowest position, pull gently down on the shorter rope, shortening the pendulum and thereby increasing its velocity. Release the rope as the pendulum nears its high point.
According to a possibly oversimplified analysis, conservation of angular momentum at the low point, before and after the pull is applied, explains why this procedure causes the amplitude of the swing to increase with time. See also discussion of parametric resonance.