I have a backup generator that always wants to crawl when left unattended and ends up getting stuck on a garage wall or some other place I don't want. You prevent it. I haven't used them to stall my car, for example when it's on a jack while the tires are spinning. To do this, I have a strong metal jack, and it is more convenient for me to trust him. But they're light, colorful, easy to spot in the garage, and good enough for at least light work.