Every pendulum has a natural vibration cycle that depends only on its length. For example, a weight tied to the end of a 10-in (25 cm) length of string will complete one back-and-forth swing in about one second.
The two pendulums in this activity have the same natural frequency because you made them equal in length.
When you start the first pendulum oscillating, it makes the attached drinking straw twist back and forth with the same frequency. Each time the first pendulum completes a swing cycle, the twisting straw gives the second pendulum a tiny shove—like a parent pushing a child on a swing.
Because the straw is pushing with the same rhythm as the natural frequency of the second pendulum, the weight swings progressively higher and higher with each tiny push.