Yeah, in a typical posi, the torque being input from the engine forces he spider gears apart... which in turn put pressure on the devices that makes the carrier a posi. In some cases the spiders back-side are shaped like "cones" and are forced into the carrier body. More commonly it's a multi-plate clutch pack on each side of the spiders. They need torque on the carrier and resistance at the axles to work correctly.

If you're on a low traction surface, and the pre-load spring on the clutch pack is weak... you can still only spin one wheel. That's where the old "Hold the brake while applying the throttle" trick comes into play. The drag created by the brakes loads the axles, allowing the clutches to grab.

The above trick doesn't work with the G80. The G80 needs wheelspin to engage. Something like a 200rpm difference from one wheel to the other.

That's the reason why I don't like them for performance use... they've violent when the engage on a high traction surface... often grenading. For average every day use, they're great. You'll never know they're there.