4x4 ATV's have the same thing, I think, yea think, it is to cushion low speed jolt when you accelerate or decelerate. Think of it as a cusion in the otherwise very direct drive system. The spring allows the gear's to almost slip, I suppose very rarely they could jump a tooth, but from what I have seen there is not room.
The video you found shows the vmax's cushion has a spring rate enhancer, so the more the power, the more spring tension there is, and the less almost-slip there is.