The idea is to put a 180-degree U-turn plus a little length of pipe so the cold air settles in the bottom to help prevent unfettered cold air flowing straight down into your basement. Just like your plumbing has traps to retain some water that prevent air/gas from flowing through.
This demonstrates why the whole concept is flawed. Unless you fill your trap with water, like a plumbing trap, gasses are free to flow all day anytime just as if there was no trap. A trap in that line does nothing for you. Think it through.
If you insist on adding this trap, another way people accomplish it is to stick the end of the OAK pipe in a bucket. Same flawed concept, treating air like water, acts as a barrel trap.