The Speed Reducer is for large diameter Holcutters or for all sizes when drilling thru stainless steel. It slows the RPMs down to a more effective cutting speed using a 4:1 gear ratio. The Speed reducer can be mounted directly to a 1/2" drill (pictured) or mounted in a 1/2" chuck using the triple-flatted adapter (04943). Holcutters mount directly to the Speed Reducer using the Quad-Lead Thread System.