It has to do with the engine speed and the speed at which the vehicle is moving. Back in times before computers it was controlled by vaccuum, the engine produces a lot of vaccuum at idle but less as you increase RPM, because you are opening the trhottle and increasing air flow through the intake which lessens vaccum to the accessories, that is why you have vaccuum resevoirs and why the cars with vaccuum control heating and AC would do wierd things when you accellerated quickly. The modulator would get the vacuum signal from the engine and upshift according to drop in pressure. Diesels are a whole different story though because their vaccuum systems are so different. Now computers do it all. But like you said if you left your engine idling, started down a hill and didn't press on the gas, the speed is what upshifts, the tranny has to keep up also as you move faster the engine will start to increase in rpm thus decreasing vacuum and signaling a shift allowing the rpm to go back down. This is done through the clutches in the transmission. With the new computer controlled transmissions, the engine speed and vehicle speed are all monitored as well as the throttle position. When you floor it, it knows to downshift. The computers also learn driving characteristics of each driver and adapt. Electromagnetic silenoids now take the place of vaccuum modulators. I hope that helps in the explanation. I am not an expert, but that is what I have learned myself as to an explantion of how it works. Thanks and have a great day!
Regards,
Brian

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Brian W. Heitman

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