A sexual act where, the female is in the doggy position with her ass as far in the air as possible, the male crouches over her facing opposite of her head points his penis down with his hand and inserts it into the females pussy, hence like the turn down exhaust on a car.
To "turn up" is to get drunk and/or high, party, and forget about life for awhile. To "turn down" would be to get serious, go to work, and deal with reality.
Turn Down For What (the song) is a social commentary making the point that many people -- especially in "the hood" -- feel that even if they do "turn down" and do everything society expects of them, the deck is still so stacked against them that their lives still will not improve.
So why bother to turn down, if there's nothing to turn down for?