Every pendulum has a natural or resonant frequency. Every time the first pendulum swings, it pulls on the lower connecting shaft and gives the second pendulum a small tug. Since the two pendulums have the same length, the pulls of the first pendulum on the second occur exactly at the natural frequency of the second pendulum, so the second pendulum begins to swing too. The second pendulum pulls on the first pendulum at just the "wrong" moments, so that eventually, the first pendulum is brought to rest. That is, it has transferred all of its energy to the second pendulum. But now the original situation is exactly reversed, and the first pendulum is in a position to begin stealing energy back from the second.