How does my auto know when to change gears?

Hi all. I'm currently driving a 4 speed 1988 Toyota Camry auto and I'm curious about its transmission system. How does it know when to actually change gears?



I've played around with it a little bit on the highways to experiment, and its a little strange. If I floor the accelerator from a stationary position (0km/h, around 1000 rpm), I'll get to about 60-70km/h and 2500-2800rpm before I can feel it click into second gear.



But if I was already moving before, say around 40-50km/h, close to 2000 revs, my auto can go over 3000rpm without clicking into the next gear. Why does it vary like that? And how does it know when to change gears?How does my auto know when to change gears?
It's pretty complicated to just explain on here, so maybe this will shed some light.