A sweep vehicle, also called a broom wagon, is the vehicle that follows the last runners on the course at the official cutoff pace. Any runner it overtakes is usually asked to stop running and board it so roads can reopen on schedule. It marks the back edge of the race, not a judgment on the runner.
It's a vehicle, often a van or bus, that trails the slowest runners at the race's official cutoff pace. It signals the very back of the field to course marshals and traffic police.
Organizers typically ask you to stop running and get on board, since you're now behind the pace needed to finish before roads reopen. Some races let you keep walking on the footpath, but you won't get an official finish time.
Usually yes, it gets recorded as a DNF in the results. The exception is if you'd already passed the final cutoff point and are simply covering the last stretch, which some races still count.
It moves at whatever pace matches the race's stated cutoff time, for example a pace that completes 42.2 km in 6 hours. That pace is usually published in the race's runner guide before race day.
Yes, simply run faster than the published cutoff pace for each timing point. Studying the course's intermediate cutoffs, not just the finish cutoff, is the safest way to stay ahead of it.