Having ridden a fixie for some time, this is incorrect. It's a lot of work to just "stop pedaling" - precisely as much work as it would take to accelerate from

Having ridden a fixie for some time, this is incorrect. It's a lot of work to just "stop pedaling" - precisely as much work as it would take to accelerate from a stop to your current speed. If you're unaccustomed to it, the force can easily lift you off your seat. If you do somehow manage to lock your pedals, you will not fly over the handlebars. Your rear wheel will skid, probably out of line, and you will slow ineffectively. You might end up under the bars, but not over. It's much more effective to stop using the front brake, which is installed on any decent fixie for the reasons listed above. Not only does it require much less effort, it is more controllable and faster because your front wheel is more heavily loaded than your rear wheel.