n0kfb said:...both wheels are powered....
HARDTRAILZ said:... they hardly ever are both powered.
Just to add my 2c to hopefully clarify the situation. An open diff, by definition, splits power equally between the two sides. The TORQUE (torque is power) sent to the left is the same sent to the right. Torque is like water in a hose - if there's a cut, it tends to flow out the leak rather than go to the nozzle at the end and do what you want it to. If it's a small leak, you might still get some decent flow out the nozzle as well.
A slipping tire is like a leaky hose. Torque leaks out the spinning tire, and no MORE than that amount of torque is going to be available at the tire with traction (the nozzle). Just like pressure equalizes in a plumbing system, torque equalizes in a differential drivetrain.
Don't confuse a tire's observed RPM with power. It takes very little torque (power) to get a tire in the air spinning freely.
One way to prove that is to get on a tippy situation with one front wheel in the air, spinning freely. Then take a ratchet strap and strap that tire to your rock sliders and prevent it from rotating. The opposite front tire with traction will suddenly be able to get some torque out of the differential, and may be able to pull you up the trail. Some times you can get part of the same effect with left foot braking, applying power and braking at the same time, and the brake-induced torque on the spinning tire gets applied to the non-spinning tire and progress is possible. Torque-controlled vehicles with automatically applied brakes might thwart this tactic.