What is auto- asphyxiation?

I was checking my son's facebook account and text messages and his friends and him keep talking about how they love to perform auto- asphyxiation (not sure on the spelling). None of my colleagues seem to know what this is and I was hoping someone here might...my son is 11 and he's really into cars...my main concern is that he and his friends are "borrowing" my car after I'm asleep and joy riding. I've kept track of the mileage but I wonder if he can reverse it, like almost happened in Ferris Bueller.