This is the starter gear. This is the starter. Between the two is a train of gears. The starter can be as small as it is because it has high-intensity rare-earth magnets in it. And it ends up driving this gear. The starter clutch is inside this housing.
The purpose of the starter clutch is to allow the starter to crank the engine in the forward direction. Yet, when the engine fires and accelerates to idle speed, it doesn’t try to drive back through the drivetrain and spin the starter at a zillion rpm. Instead, it releases.
One thought you might have is, “Oh, a ratchet.” We put one ear down to it… It’s completely silent. No clicking. So it’s not a ratchet; it’s a roller clutch. Inside this hub are a number of little ramps. On each ramp is a roller and a spring, which presses the roller inward.
When I try to turn the gear away from me, the rollers are driven so hard against the crankshaft that they grip it and turn it. But the slightest motion in the other direction pushes the rollers back up their ramps, out of contact, and the clutch releases.