Originally Posted by Kaldaien

There's a device that plugs into the OBD-II port on the car. It measures the speed every few ms and detects changes in it. They only care about quick deceleration (> 7mph/s), which they call "had braking" (which is silly, because you can do that with 1 bad downshift). But the data recorded includes your speed, they just aren't allowed to do anything with that

As for how they get the data, Progressive has a contract with AT&T and the device sends its data log wirelessly every time you start your car.