An electric knife uses electricity from a 110-volt wall outlet to run its motor. Like any other appliance that plugs in, it grabs the electricity from the wall using a thick cord with two prongs on the end of it. The cord consists of both positive and negative wires that carry both the power current and the ground connection necessary to complete a circuit.
Inside the electric knife is a motor that spins when the 110-volt electric power is applied to it. These motors are sometimes called "rotary" motors, because they use a circular motion to transfer power to a certain device. They can be found in common household items such as drills, reciprocating saws, grinders, blenders and of course, electric kitchen knives. Rotary motors inherently use quite a bit of amperage, so it is a good idea to make sure that the circuit breaker on the panel is rated at or above the requirements of the electric knife. The electrical usage and specifications can usually be found somewhere in the user's manual or instructions page. Also, try to plug the knife into an outlet not used by any other appliance so that it can get full power safely.
When the electric rotary motor spins, it actuates a linkage that is internally connected to the blade of the knife. As the motor turns, it causes the blade to rapidly cycle back and forth. That rapid speed is how the knife cuts through things so easily. The sharpness of the serrated edge coupled with the revolutions of the rotary motor make for a wonderful combination in a cutting tool. The revolutions per minute of the motor can probably be found in the user's manual, as well, if interested. They will vary from knife to knife.
As mentioned, an electric knife has the ability to cut through tough objects very quickly and efficiently. Use extreme care when operating the unit, because they can cause great injury. Stick to the manufacturer's recommendations when cutting. Do not attempt to cut through metal, plastic or any other object or substance that the knife was not intended to pierce. Keep the knife away from children, as well.