What *IS* the Show All Tabs Gesture?

Occasionally I unintentionally trigger the Show All Tabs window in Safari with my magic mouse. I can live with this, but the frustrating thing is I have no idea how I'm doing it. What is the gesture that opens this window?

Some sources say it is a two finger pinch, but I have tried ad nauseam and this isn't it. At least, not on my system. I have tried a whole bunch of other possibilities, but I can't intentionally reproduce it. I can only accidentally trigger it.