The poem imagines a body that has come apart, with its pieces no longer feeling like they belong to the same person. The foot looks at the hand and sees something annoying and out of place, an "impertinent echo," as if the hand were a cheeky imitation of itself. Calling the hand a "gone member" makes it sound like someone who has left a club, or a limb that has been lost, even though it is still there.
The last two lines are the strangest. The foot is "thinking how to stand," which suggests it is trying to hold itself up and work out its purpose alone. The final phrase, "a thing on whom no being went," seems to say that nobody is really inside this body, that no self or spirit lives in it.
The mood is eerie, cold and a little darkly funny. The speaker seems detached from their own body, watching it like a stranger. The title, "Extreme position," fits that sense of being pushed to a limit.