I read about it in the Park Tool repair book. I guess the basic gist of it is putting a 10mm box-end wrench around the axle on the side of the hub that you're not adjusting, then tightening the skewer around that to simulate the tension and pressure on the bearings when the wheel is clamped into the dropouts. The skewer tension holds the wrench against the locknut so it should turn with the axle/locknut/cones on that side while you adjust the other side. Then, you can also use the wrench as a lever to push/pull to check for play while you're adjusting the cones on the other side. Hopefully that makes sense.