Like a variable-speed hand drill, your backyard power hammer should be able to run slowly, at full speed or in between, depending on the task. The simplest way to guarantee it does this is to use a slipping clutch.
There are two main designs of slipping clutch. One uses a slack belt, a flywheel and pulleys. The drive pulley rotates within a slack belt. The foot pedal linkage pushes an idler wheel into the slack belt, increasing the belt tension to the point that the belt begins to turn the pulley on the flywheel.
Another design uses
