If given a bit of time in I could create a cad model that demonstrates exactly how this works more clearly to those confused, but here is the basis with a bit more detail:

The motors for this system both power the wheel and/or turn the module. If both of the gears are driven at the same speed in opposite directions the module remains in the same position but the wheel spins. If the speed of one motor is greater than or less than the other motor, the module will rotate, while the wheel still spins at some rate (the wheel speed is related to the speed of the upper gear plus the negative speed of the lower gear).

To help with understanding, if you want to simply rotate the module in place without spinning the wheels you must rotate the gears in the same direction rather than opposite, and at the same rate. Any difference in rotation rate between the upper and lower gear will cause the wheel to spin.

For those who like equations:

R=rotation of module

S=speed of wheel

V1=speed of top gear

V2=speed of bottom gear

K1 & k2=scaling coefficients (because these aren’t 1:1 ratios)

R=K1(V1+V2)

S=K2(V1-V2)

The signs are very important in these equations. You will notice for there to be no rotation of the module the speeds of the two gears must be equal in magnitude, but opposite in direction.

If you want the wheel to be rotating while also spinning the module, the speed of the two gears must be different, and must not sum or difference to zero, but the total speed of the wheel is determined by the total difference in speed of the gears.

I probably confused more people than I helped understand, and I probably went way too in depth, but I hope this helps someone understand this ingenious idea.