At rest the rotary dial was at say null. As soon as you moved it over a number and then released it, that was the number that was dialled. 0 and 1 were first (obvs). When you hit the backstop (a small metal shim) then you had hit nine. It isn't as bad an interface as it might sound. You would get feedback via a series of clicks as the dial returned to null and also via force (springs etc).
There really was a good reason for picking 999 originally and the world has moved on since but I still assert that 999 is a far better number (or nowadays: a symbol) for invoking an emergency call than any other number.