What do headers do?

Headers by design, smaller tubes at exhaust ports, actually create a vacuum which pulls spent gases from exhaust ports much faster than standard manifold that just lets gases flow through exhaust. In turn headers create more pull of air into intake usually increasing horse power and torque. Size of header tubing, smaller usually improves performance in low and mid range rpm, while the larger header tubing usually creates more power in upper high end of power range. Hope this helps.