While in the 2stroke world jackshafts are usually used to transfer the engine power to the pedal drive side for shifting, it can also be used for an extra form of gearing reduction to not have to use an unreasonably large rear sprocket or just to offset the sprocket location on a wide motor (or wide rear tire) to line up with the other sprocket. I made a 3/4" jackshaft for my bike to both offset the wide engine and add a bit of reduction, and as an added benefit it also makes for cheap and quick gearing changes and fine tuning without having to pull the rear tire - just remove and replace the small shaft sprockets to change gearing. Just like with the engine sprocket on a chinagirl, going +1 on the jackshaft input sprocket is the same as almost +3 on the rear and +1 on the output sprocket is almost -3 on the rear. A small jackshaft sprocket is not only easier to swap, but also like 1/3rd the price of a decent rear sprocket.