Autofocus clutch. The bigger gear at the top is driven by a small pinion attached to the "screw driver" in the lens mount. It is attached to a small dog clutch, which drives the smaller output gear which engages the gearing on the gray main focus barrel, which in turn focuses the lens.
<a href="//commons.wikimedia.org/wiki/User:Phiarc" title="User:Phiarc">Phiarc</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>