The ride uses hydraulic pressure to spin a drum and cable that launches the coaster train to 120 mph. More information about how it works can be found
here.
It is designed to complete its circuit (and thus, go over the top) with plenty of speed to spare. It doesn't stall in any sense except for when environmental variables throw the proverbial wrench into the works. For example, outside temperature and wind will each have an effect on how the ride operates, and so Cedar Point must accomodate those conditions by shifting the trains center of mass to its most advantageous position. Sometimes, that's why they'll only load the first half of the train. Top Thrill Dragster is far from being the only roller coaster that operates this way. In addition to TTD's sister rides, Xcelerator at Knott's Berry Farm in Buena Park, CA and Storm Runner at Hersheypark in Hershey, PA, most other roller coasters need operational adjustments based on outside conditions. When I rode Superman: Ride of Steel at Six Flags America on a 33* October morning, they also had to load only the first half of the train so it would complete the circuit, for example.