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With a normal reverse gear linkage, it lets you set up the valves so that the steam inlet is cut off partway through the stroke - the steam will keep expanding through the stroke, but you are not wasting any more by keeping the inlet open. With an engine under light load (or none) you can save a lot of steam that way - under a heavy load they would have to open the timing up to keep it open longer. The Stephenson type linkage lets the operator change that setting on the fly. For the case where you are reversing the inlet/exhaust, you would have to keep the timing set more to the open-longer case.