That would be the spur gear and the clutchbell gear. I've never had mine get anything in it that actually stuck and caused a problem. If you're running in gravel you may have more of a chance of that happening though. On my 1/10 scale truck I have had rocks in the track hit the flywheel when I land a jump and stall the truck. Has only ever happened twice in the last five years though.
Are you having problems with this happening or just wondering if it is something to worry about?