Foot pedal, behaves like a dimmer, but with variable speed. The more you push, higher the speed, foot off == machine off. On the machine itself you would put it into max speed, so this thing can work correctly. The machine speed knob will act as max speed limit knob, when used with this pedal.
It is very popular, people use it with dremel. I want to use it with my desi dremel == Ingco mini grinder.
In the west it is easily available for 110v, we need the 230v version for our 230v machines, hard to find here for the right price.
Here the momentary version of this sells, which is basically a simple ON/OFF control through the foot, no variable speed.