[leereeves]

I was able to consistently start a loop in one click by clicking on the edge between grey and white (slow and fast refractory) cells ASAP after a normal beat.

If I'm fast enough, the wave can't spread to the grey cells so it only goes in direction around the red cells and starts a loop.

(But you can't do this on the first image with a Defibrillate! button because there are no slow cells.)