Donne's poetry is remarkable for the way in which he combines reason with emotion: even in his most passionate love poems, the reader cannot let her or his brain cells rest. There is always a thread of thought which must be followed if the poem is to be fully appreciated.
Sometimes he extends his ideas to the extreme, a quality noticed by Dr. Johnson who commented on the Metaphysical poets: "If their conceits were far-fetched, they were often worth the carriage". By this he meant that there would be a truth at the end of the journey of understanding their unusual thought or fancy. In literary criticism now, the term, "conceit" is used to denote a comparison between two unlikely things, stretching credulity but expecting the reader to admire the ingenuity of the writer.
The best example of this is Donne's poem, "A Valediction forbidding mourning", in which he argues, somewhat unconvincingly, that physical absence between true lovers is unimportant because they are "Inter-assured of the mind". Listing what will be missing (eyes, lips and hands) only makes the reader realise that he is trying to persuade his partner not to mourn his departure - whilst being uncertain himself.
This sense is compounded by the ultimate motion in the image of the lovers as "stiffe twin compasses" which are two in one. She is the fixed foot which moves only when he does, leaning further outwards if he moves a greater distance away, growing "erect" when he returns.
Yet, if you take a pair of compasses and enact the final stanza, it says the opposite. The conceit works against the emotion. The feeling is that the lovers will be reunited but rationality has them apart: her firmness and constancy cause him to draw his circle "just" and end where he began. That means that he has joined up with his own trajectory and drawn a perfect circle - but is still apart from her. There are two different motions in those three stanzas: the first where the centre foot (her) draws the wandering one back to it; the second where it (she) stays firm, guiding his circle but ending with him still away from her.
My reading is that the persona of the speaker is doubtful of his own argument: the latent eroticism reveals that uncertainty - as does the final stanza of the compasses conceit. He fears that they could stay apart after his absence.