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Power Ratchet, Power Shifter
"Power Ratchet" is an advertising buzzword for a Sun Tour design, that decouples the shift lever from the friction clutch when shifting in the direction that pulls against the derailer's return spring.
With a pure friction lever, the friction has to at least equal the maximum pull of the derailer spring. When these are close, it takes almost no effort to push the lever forward (loosening the cable and the spring,) because the spring is helping you push the lever forward.
However, when you pull backward on the lever, tightening the cable, you are working against both the resistance of the friction clutch in the shift lever and the spring tension.
This creates an asymmetry of lever force between upshifting and downshifting.
With a "Power Ratchet" system, the ratchet disengages the friction clutch as you pull the lever backward, so you're only working against the spring. This gives a notably nicer feel to the shifter.
Other brands have solved this problem different ways. Shimano used to use a spring in the shifter that opposed the derailer's spring. This avoided the slightly "granular" feel created by the ratchet.
The legendary Simplex "Rétrofriction " levers did something similar, using a pawl-less ratchet. As a result, the Simplex design didn't have the "micro clicks" of the Sun Tour design.
