Asking an exam question that requires a specific technique
You say, "Solve for Y.  You must use X technique to demonstrate that you know how to use it, even though there may be simpler other ways to solve this particular problem."

Edit: One should cover in class why the students need to know X.  The point in the exam question is to make it clear that using X is a requirement of the question, not a hint.

Example: "Use a Karnaugh map to simplify the following Boolean expression."  Here, the question asks, "use a Karnaugh map," not "simplify the expression."  The latter is the mechanism to demonstrate the former.