[MetalMan]

I'm not Brian, but...

That middle gear is typically referred to as the "idler" gear. It's named that way because it doesn't affect the overall gear ratio, it merely reverses the direction of the output and puts distance between the input and output gears. For example, the Rustler transmission has a 22t top gear, 30t idler, and a 60t differential gear. The overall ratio is about 2.72:1, and if you calculate out the ratios:
22/30 = 1:1.36
30/60 = 1:2
Multiply those two ratios together, and you get 1:2.72 (the same as 2.72:1, I just flipped the numbers in this case).

The same can be accomplished by the first and third gears:
22/60 = 1:2.72