The basic idea is simple: put out a radio magnetic pulse, vehicle electronics get confused -particularly the ignition -, car/bike/truck stops.
The main application touted is the securing of buildings by the police etc., but it doesn't take much imagination to come up with other uses. It's mostly meant to be a safer alternative to spike strips.
However, as the BBC notes in its article, it wouldn't work on older vehicles:
"Certainly if you took a 1960s Land Rover, there's a good chance you're not going to stop it,"
Cue senior citizens smirking grimly while adjusting the points on their Ford Gran Torinos...