Here is a rhetorical analysis of that sentence from Thoreau's speech:
Thoreau uses very powerful and direct language in this sentence to make his point about resisting an unjust law like slavery. By saying "If one HONEST man...were actually to withdraw from this co partnership", he emphasizes the moral high ground of refusing to support slavery through taxes. Describing withdrawing from the "co partnership" of the government frames it as a business partnership one can leave.
Saying the man would be "locked up in the country jail" vividly illustrates the penalty for dissent, evoking an image of unjust imprisonment. Using the specific location of "the country jail" in Massachusetts gives the scenario immediacy and realism.