It's basically having the rear act like it was a solid axle without the differential. This is accomplished by filling the differential with glue to lock it.
From a post of mine elsewhere:
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Since the AWD kit comes with an extra gear diff, I felt perfectly safe gluing my differential down. I used hotglue, because it still is removable if necessary. I stuck the nozzle in between (the middle of) the center gear and the outside edge, and between each cone gear. When I pulled the hotglue trigger, it stuffed hotglue in between the gears, and partially under them. Then I pressed the glue flat with my finger (still hot) until it cooled just enough to release my finger (still moldable), then screwed the differential top on. Here's the result:
See the ring of reflected light around the center gear? That's from the inside of the differential cover. The blue blurs are the cone gears.
