How does a cordless impact driver work?
Whereas a drill simply puts power to the motor and drives it in a constant, linear rotational motion, an impact driver has much higher torque and applies the force in a different fashion.
When you pull the trigger on an impact driver, a force is applied in a rotational "hammering" motion, or, an "impact". Broadly speaking, it's like having a hammer inside the motor, pounding out rotation and a few thousand times per minute.
This makes for a tool that is highly effective at driving screws and lag bolts.