In the ebay link, that is the piece on the close side of the spur gear in the first picture, in between the gear and the nut/spring. That is the slipper clutch. It's job is to save the gears from stripping when they started spinning fast and very suddenly (like when landing a jump). The clutch with clutch shoes attaches to the engine and it's what engages the gears to get them moving to propel the car. I'm not sure if this is what you were asking about or not, but I did my best. They do two separate jobs.