The cordless impact wrench is quickly gaining in popularity among professional mechanics and the hobby mechanic as well. You may be thinking "I already have a cordless drill that I use for impact applications, why do I need a cordless impact wrench as well?". I thought the same thing. The first difference is on the inside. A cordless drill creates continuous in-line torque. A cordless impact wrench employs an internal spring-loaded pulsating cam to create rotational torque. This creates the "impact" force that loosens a nut. It is the equivalent to banging a wrench with a hammer loosen a rusty nut only a lot faster. Unlike a cordless drill - which creates continuous in-line torque, a cordless impact driver uses an internal spring-loaded pulsating cam and gear mechanism to create rotational torque or the "impact" force.
Another big difference, is the lack of reactionary torque. Reactionary torque is what you experience with your cordless drill. You have to use brute force to keep your drill from spinning the opposite direction in your hand. With a cordless impact wrench, that torque is directed to the nut not your arm. You can easily hold this tool in one hand and let the cordless impact do all the work. No more pushing down on the tool to drive that screw. These tools are also smaller and lighter than a cordless drill. For example a DeWalt cordless impact wrench is about 35% the size of its comparable cordless drill. This feature makes them perfect for those overhead projects and reaching into hard to get to spaces.

The next time your start that big deck project in the backyard, leave the cordless drill in the box and grab your cordless impact wrench. Your arm will thank you at the end of the day.