Self-propelled mowers are not a new type of mower, but an improvement on the rotary mower. Like other rotary mowers, self-propelled mowers use a spinning blade to cut grass. This spinning blade is mounted horizontally beneath a protective deck and is powered by a gas motor. The blade cuts grass and either spits it out the side or into a bag or else slices it into tiny pieces and leaves it on the ground as mulch.
While other rotary mowers require the user to push the heavy device across the lawn, self-propelled mowers have a type of power assist built in. The motor drives a gearbox which turns the wheels of the mower at the same time it spins the blade. This allows you to walk behind the mower and steer it with relatively little effort, decreasing your workload.
Many self-propelled mowers have one, two or several set speeds. You can set the mower to the speed closest to your natural walking speed to make mowing more comfortable. The more speeds, the more closely the mower can match your walking speed. Some self-propelled mowers go a step further with continuously variable transmissions. These mowers sense the speed you are walking at and match the mower speed to it.