It’s what the spur gear (big gear on transmission) is attached too.
There are pressure rings on either side of the spur gear that get compressed against the friction pegs (red pegs in the spur gear) by two pressure plates that the spring and nut apply pressure too.
To clean it you would need to disassemble it, to adjust it tighten the nut till the spring is compressed fully and then loosen the nut about 1/4 turn. Adjust from there for a tighter slipper or a looser slipper.