This is an illustration of how back EMF is produced in a motor. In this case, a power tool. If you look in here, watch when the motor stops and you'll see a spark inside that you can see through the vents for the heat, and you can see how much shock there is produced when the motor stops. See that bright flash when the motor stops is the back EMF. That's produced because there's a big current flowing through the motor when it's on, and then when it stops suddenly, that produces a big back EMF as the inductance tries to keep the current flowing.