Here is my rendition...
(I know that everybody knows this and I know that you are not stupid, but I am just saying it for the sake of clarification and the description. Just so that everybody knows were I am coming from.)
You know how when you spin you pedals in the wrong direction on a bike they just spin? No power is delivered to the rear tire and you don't go anywhere. That is as long as your bike isn't a fixie or has a roller brake.
Well that works because you have a freewheel or cassette system on your rear wheel that has a racheting system that only allows power to be driven in the right direction. This allows you to coast on your bike without having to pedal all the time.
So, I guess, one manner of rachetting mechanism is the sprage or sprauge or however you want to spell it. Apparently Wikipedia says these things are common in UK engineering and in automatic transmissions. Something like that.
So here is my version... The black circle represents the input shaft. The yellow is the output.
The goal is to spin the yellow shaft in a clockwise direction.
So there are two gears attached to the yellow shaft. One is the Red gear, and the other is the blue. They are not keyed to the shaft, instead they are pressed onto two sprauges. So that they can spin independently and can only apply power to the yellow shaft in a clock-wise rotation.
So on the black input shaft there are two gears, the Blue one and the Red one. These are keyed to the shaft so that they don't rotate independently... they are fixed to the input shaft.
So when the black input shaft rotates counterclock-wise this causes the Small Red gear on the output shaft to rotate clockwise. This causes the sprague to engage and drive the shaft.
At the same time this causes the Blue gear on the output shaft to rotate counterclockwise. This causes the sprauge to not engage on that gear.
So when the black input shaft rotate clockwise this causes the small Red gear to rotate counter-clockwise and thus disengage.
And, finally, when the black input shaft rotates clockwise this causes the Blue gear on the rear output shaft to rotate clockwise, engaging the sprage clutch and driving the yellow ouput.
So input rotating clockwise == high gear.
Input rotating counterclockwise == low gear.
I guess the common spelling is 'sprag'. Here is a example of a 'sprag clutch' or 'backstop' in the form of a roller bearing:http://www.vxb.com/page/bearings/PROD/Kit8181