A kid I went to school with (white guy, if it matters), referred to it as “pimpin’ it.” Slouching way down low in the seat, usually with one hand on the wheel.
Back in the 70’s it was leaning as far to the right as possible. I remember following a guy and all I could see was the top of his head between the bucket seats. With all the crap he had hanging off his rear view mirror, I was wondering how he could see where is was going.
No, I haven’t. They’re always in “street wear”…nothing dressy.
They usually seem to be driving a 10-15 yr old widebody sedan…
I think the other posters have nailed it as the “gangsta lean”. Only problem is, I don’t understand how doing this is “cool”. Then again, I’m 37, so I guess I’m too old and un-hip to understand.
It’s probably one of those things that if you have to ask, you’ll never understand. 
Two things you’re unlikely to see: a white guy doing the slouch, and a black dude riding a Harley.
I’ve seen both in the last month, but white guys doing the lean are way more common around here.
Don’t be hatin’…
[Homer]It’s true! We’re so lame!!![/Homer]
60 Minutes once did a piece on Wynton Marsalis doing a mentoring program for promising young musicians. One scene featured him conversing with a 13-ish kid, to whom he addressed the following:
"Ah, you’re being “cool”… You know what the definition of “cool” is? It means, having a limited emotional range.
"Like, if someone tells you you’ve won a million dollars (shrugs non-challantly). Or, if someone tells you your best friend got hit by a bus (shrugs non-challantly).
“You don’t need to be cool with me”.
Adolescent’s slouching is a way of displaying a relaxed “I don’t care” attitude without having to say anything verbally. Showing a relaxed disinterest in the outside world is a form of rebellion, which in turn is perceived as “cool” by other kids and annoying by us adults. You do remember the age when being rebellious was cool, right?
The opposite of this slouching driving posture would be a young person sitting ramrod straight with hands gripping the wheel at the 10 and 2 o’clock position focusing with intensity on his driving. That kid might be considered an anxious geek with no self confidence (Waldo, you really need to try to make friends this year…)
BTW, I have seen guys driving lowrider cars slouch like that for as long as I can remember (early 70’s).