The double arrows on the far left and far right increase or decrease the speed at which it steps through the code. If you click them it enough times it will actually start to go in reverse.
It seems a bit buggy though and doesn't always have the same behaviour. Might need to press stop then play or just refresh the page.
This is not a complete answer. From what I've done so far:
I believe that the problem might lie in the initial conditions that you've set
there is also a numerical problem or algorithm problem
I had to modify the code to simplify it to only two variables to do some basic checking. (See below).
For the parameters I've set:
m1, m2 = 1
k1, k2, k3 = 0.5