This is a brilliant illustration of a consequence of conservation of angular momentum. Even if the spinning mass is made to be very small, it is enough to resist being pulled over by the falling mass. As the string gets shorter (small r) the angular velocity must increase and so does the tension in the string.

Energy shuffles between the rotational kinetic energy of the spinning mass and the potential energy of the vertically moving mass.

I don't quite fully understand all that is going on, but I find it amazing that we can learn physics from js simulations... who needs a big lab for experiments when there is the browser?

Props to @dllu for coding this.