The impact drivers I am speaking of are battery powered, and resemble cordless drills in appearance. The most obvious difference is that the 'chuck' is usually a small collar, that will only accept a 1/4" hex shank.
In operation, they act like a drill ... spinning the bit quickly ... until resistance is encountered. Then, an internal mechanism takes over. A rapid series of little 'hammer blows' are applied to the rotation of the bit, allowing the bit to rotate a little with each blow. In use, these impacts sound more like a ratchet than anything else.
Little or no torque is felt in the handle.
Virtually every maker of cordless tools has a model. Milwaukee even has a 'super size' model, intended for use with their 'selfeed' large diameter forstener bits. Some even have a handy LED light in the nose.
