The Shimano single speed coaster brake hub is a lovely little piece of engineering. Peddle forward and the driver unit turns, which screws the clutch up the screw thread. The serrations on the end of the clutch unit "bite" into a narrow track inside the hub body, providing forward motion. Turn the cranks backwards, results in the driver unit screwing the clutch unit in the opposite direction, which has the effect of disengaging the clutch and simultaneously pushing the two brake shoes outward and into the hub shell. All very simple and effective.
Problems arise when the small serrations on the clutch unit become worn down and the clutch struggles to get enough bite onto the hub shell, so meaning forward drive is sporadic and the cranks often slip. Luckily, they only cost 8 Euros to replace so today was replacement day. I also purchased two new brake shoes and replaced those at the same time, again, cheap as chips.
A bench vice really makes the job much more straight forward and after cleaning and re-greasing everything, it all went back together nice and quickly. A quick spin up the road made me realise I need to re-calibrate my back peddling technique, these new brake shoes can induce a pretty mean skid if you over do things!