With an open differential, both tires put the same (or essentially the same) power to the ground in all scenarios. The tire with the least traction is the limiting factor.
So if one tire is spinning and one is not and the rig is inching forward, both are acutally putting a small amount of power (essentially equal) to the ground.
If you think about it, its pretty easy to understand. If one wheel is completly off the ground, neither tire moves the rig forward. If one wheel has only a tiny bit of traction like during a burnout, both tires aplly power the ground to move the rig foward, the amount of power put to the ground is limited by the traction of the tire thats spinning since it has the least traction (and on an open diff its usually on the side with less vertical force due to the rotation of the driveline and weight distribution). Increase the traction and increase the power to the ground.
