Yeah it works really well in the manifold. The way you clean the injectors is you add it into the gas tank, you can also add it into the oil to clean lifters/valves, and the way you clean the manifold is by pulling off a vacuum line, and sucking up seafoam in the line. If you do this your car will spit out gratuitous amounts of smoke for a a few minutes, that's the deposits all being burned off and cleaned out from the manifold, combustion chamber etc.
I've had great experiences with it I really like it.