In most respects, self-propelled mowers are identical to other gas-powered rotary mowers. Self-propelled lawn mowers are generally powered by single piston gasoline engines mounted so the shaft is positioned vertically. The shaft is attached to a rotating blade that spins at a high speed, cutting the tops off grass blades. Depending on the rotary lawn mower, grass blades are either sucked into a bag, spit out the side of the mower or sliced into fine mulch to fertilize the lawn.
Unlike other rotary motors, however, the engines in self-propelled lawn mowers are attached to a drivetrain. As the engine spins, it turns a gearbox which turns the wheels of the motor. Gasoline lawn mowers are quite heavy, so having a self-propelled mower makes it much easier to cut the lawn, particularly if you have rough or hilly terrain.
Many modern self-propelled lawn mowers have built in gear systems. Although you can't walk at any speed you want, you can choose between up to six different speeds on these systems, making for a more comfortable mowing experience. Other self-propelled lawn mowers go one step better with continuously variable transmissions. These transmissions sense your walking speed and match the speed of the mower to match it.