Thanks for testing it! The idea is to change the length at twice the frequency of the pendulum. With this you can pump energy into the system. This gets more difficult for an angle of 90 degree elevation, as there the pendulum resonance frequency changes rapidly. I got a sumersault once, but I am not 100 % sure if it still works as it is sometimes quite laggy on my system. I would need to do more in depth analysis on what is causing the lags. I took a lot of inspiration from the tetris game in emacs, which runs without laggs. I tried to synchronize the physics with the game loop by measuring the system time, the thing is if the game loop is slow, the physics loop needs to update a lot, which might cause further lags.