Sooo, it's like a torque converter?
Not at all. All a jackshaft does is allow for compound gear reduction.
Think of it this way. If you want a 10:1 gear ratio, and you just use a clutch and axle sprocket, you are going to have to have a combination something like a 10T clutch and 100T axle sprocket. A 100T sprocket is unrealistic and will probably drag through the dirt cos it is so big. So what do you do?? You use a jackshaft. With this, you can use smaller sprockets all around, and still get the 10:1 gear ratio without scraping your a$$ through the dirt.
You could have a combination like 10T on the clutch going to a 25T on the JS In side (this will give you a ratio of 2.5:1). You can then have a 14T sprocket on the JS OUT side going to a 54T on the axle (this gives you 3.85:1). If you multiply these ratios, you will get a 9.625:1 ratio. You can use all sorts of sprocket combinations to get whatever ratio you like, and the sprockets can be a lot smaller that if you just had a clutch and axle sprocket
Some pics of my minibike jackshaft below, but it is the same concept for a kart
