The yin-yang of in-house legal counsel appeals to me as a metaphor, and there are others (See my post of Nov. 7, 2010: metaphors with 25 references.).
Law departments slash with a “sword” that asserts the rights of the company while they brandish a “shield” that protects the company from legal attacks. Alternatively, the “carrots” of law departments lead business units to make money within the constraints of the laws: the “stick” of obstacles and unacceptable legal risks do likewise.
From a more complicated metaphorical perspective, ignore the meaning of “checks” as stopping something and redefine them as drafts for payment. With that, law departments are “checks” as positive contributions and traditional “balances” of guiding the business away from unknown or excessive legal risks.
Finally, in-house counsel separate the “wheat” from the “chaff,” the key legal issues and how to mitigate and navigate them from the hurly-burly noise and minor issues of a transaction.