What is a coaster brake?
 

I've been into fixed gears but couldn't quite figure out how coaster brakes worked on my bike as a kid. I still have no idea really, I see ones you attatch to a singlespeed frame and others are built into the hub? I might need one to build a bike for my friends to learn how to ride with me. Just how do they work?

Buy a $5 Kid's bike from a local thrift store, and take it apart.

Going from memory, and perhaps not having all the names right.

There is an anti-rotation arm tied to the chainstay. Perhaps not 100% vital, but the forces generated by the brake are much more than say a 3 speed multi-speed hub that may simply use slotted axles for anti-rotation.

Tied to the sprocket is a threaded spindle, so when when you pedal forward, it locks the hub to the chain sprocket. When you pedal back, some internal part (cone?) unscrews along the spindle and forces bake shoes up into the outer hub shell to slow the bike, or even lock the wheel.

I don't know about larger foot brakes.

https://www.google.com/search?q=coas...GVRDA4XU5kJ5M:

"A clutch mechanism allows the hub to engage and drive the bike forward. The clutch then disengages for coasting or braking. Rotating the pedals backwards expands the brake shoes into the steel hub in order to slow the bike. ... When the pedals stop turning, the clutch pulls away from the hub shell."