a jackshaft is one of life's great mysteries, one that the gurus tend to forget to describe in detail to the new guys who can't take the time to google it or search for it on the forum. jeez. lol.
a jackshaft is simply a mini axle with sprockets and often the driven pulley from a torque converter setup. the jackshaft on my large kart serves 2 purposes: gear reduction (a small sprocket feeding the rear axle sprocket, and a slightly larger sprocket fed by my driven pulley's sprocket), and also serving the purpose of allowing me to keep my motor centered in the rear of the kart, while transfering the torque of the chain to the rear axle sprocket located closer to the bearing - so the axle doesn't bend from the torque of the motor pulling on the chain.
a jackshaft is a cost effective and time saving way to gear your kart down for more torque (or up for more speed).
Tip of the day: you never want to put your sprocket in the middle of the axle! always put the sprocket near a bearing - so the axle doesn't bend when you punch the gas or climb a hill.