Burnout CRASH!

An explosion chain is where one car explodes and sets fire to another car. When that car explodes you get a 2x chain. Now if that car sets fire to another car you get a 3x chain, and so on.

This is an achievement that you'll probably pick up during the game anyway, but if you're looking to complete it quick I can give you a strategy.

Play rush hour and create a pile up at an intersection. Don't use your crashbreaker once cars start driving into each other, they will damage each other and eventually one will catch on fire. If you have a large but quite scattered pile, that car should start a explosion chain. It's mostly about luck if you get a big chain going, but this method works (I've used it on events that require chains).