Test plans. Unit test plans. When you hand off the duties of writing a unit test plan to someone, doesn't it make sense that you would actually have something in place, like maybe a set of instructions or some samples or something to give him as well? Or perhaps even having a process in place?
The most explicit directions in the document template saying "there are elements of white box as well as black box testing, and some familiarity with the code is assumed" will not provide you with the kind of detail or even the direction that you want to go. And when you get into a document review of this poor guy's first test plan, don't chew him up and spit him out because your process is unclear.