If you’ve ever wondered what those chains are you see hanging from emergency vehicles, there’s now an answer for you.

They’re built-in chains in the case of inclement weather.

These “automatic tire chains” are sometimes referred to by their brand names, like Onspots or Insta-Chains. They hang there, not really doing a whole lot except waiting for bad weather to appear, but when it does, all the driver has to do is flip a switch to engage them.

The chains lower until they hang in front of the rear tires, spinning in front of the wheels. As the tires rotate, they catch the chains. This greatly improves traction on icy or slick roads, preventing accidents and pileups and generally reducing chaos wherever they go.

These types of vehicles usually don’t have time to spare or waste, which makes the automatic nature crucial, and since they can sometimes struggle driving uphill on slick roads, the chains are a snap.

If you want to find out more, watch this video.