When a bong session is moving too fast for a weaker member to keep up. Derived from the british word, stacker, and designed as a play on words with the Dakar Rally.
D: Mate, slow down on these bongs a bit. I'm wrecked.
B: Nahhh man, I'm gagging for a tube.
D: Go on mate, its like the shtackar rally or summet.