This project uses a pair of differential gears to create a transmission that has an infinite number of gear ratios and automatically cycles through them as resistance on the output changes. It's gear ratios range anywhere from 1:3 (Output spin 3 times as fast as motor) to a sort of powered neutral gear where the output doesn't spin but the motor spins and still applies torque (Similar to an automatic transmission car stopped at an intersection).
I discovered later that this sort of a transmission is usually called a CVT - Continuously Variable Transmission, but as far as I can tell IVT - Infinitely Variable Transmission is an interchangeable term. I also found out that others have made similar Lego CVTs using two differentials, though I didn't copy them at all (I found the others after I made this) and I think mine is the only one where how it works is explained thoroughly.
Lego Automatic Infinitely Variable Transmission
Feel free to use this in your Lego creations, I know I will. I'd appreciate it if you'd mention me in the credits though. =P