It is when you are in the backseat of a car and throw up into the pouch on the back of the driver's or front passenger's seat and then smack it, sending bile-filled vomit all throughout the cabin.
"Dude, why is Pepe pissed at you"
"I was hammered last night in the back of Don Pepe's car so when I had to vom I gave him a Backseat KamiKaze."
When you break wind and it smells so bad you start to gag. Usually happens while driving or in a small room.
Can also be used to attack a group of people by standing near them and farting loud.
Dude 1: Jesus, I had a kamikaze fart driving home. I almost died.
Dude 2: Shit nigga.