Spivak is postcolonial theory's most demanding voice, and the one who turned its critical apparatus on its own practitioners.
She entered the field as Derrida's English translator — her introduction to Of Grammatology (1976) was for many readers their first serious encounter with deconstruction — and the training shows: her characteristic move is to find the term a discourse cannot examine without unravelling.
"Can the Subaltern Speak?" (1988) is the essay everything else is read through. Its argument is that the subaltern — the people at the very bottom, outside the structures through which claims are made — cannot be simply "given a voice", because to be heard at all they must speak in an idiom supplied by the institutions that constituted their subordination. Her central case is the debate over widow immolation in colonial India, where the British represented themselves as saving brown women from brown men and the nationalists insisted the women had chosen; between the two accounts, she argues, the women's own position is not merely unheard but unlocatable. The essay is at least as hard on radical Western intellectuals — Foucault and Deleuze are her named targets — as on colonial administrators, for imagining that the oppressed can speak transparently through them.
She coined "strategic essentialism" for the necessary compromise: a group may need to assert a unified identity it does not really possess in order to act politically, provided the fiction is held knowingly. She later distanced herself from the phrase, finding it had become a license rather than a discipline.
Her prose is famously difficult, which she has defended as a refusal to make the argument consumable on terms it exists to question.