الْمُرَادُ بِصِيغَتِهِ مَا دَلَّ عَلَى طَلَبِ فِعْلٍ
The relevant imperative form signifies requesting an act.
The basic meaning of a form differs from its meaning in a particular utterance. Especially in figurative and indirect usages, the grammatical form remains while the intended force changes.
Thus iʿmal addressed to a student can genuinely direct work, while iʿmal mā shiʾta in a threatening context means “go ahead, but face the consequences.” The words preserve the imperative structure, yet their rhetorical purpose is different.