Eli5: What is the purpose of downshifting for a semi-truck and how does it work?

Question

I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?

In: 547

Answer 1

Jake-breaking is what they call it some places. You don’t have to touch your brakes…. I used to jump from 3rd gear to 1st sometimes in traffic, in a passenger car, as 2nd gear is worthless in rush hour.