Indeed, puta occurs in the imperative as an idiom that sort of literally tells people what to think, or imagine: it's used to introduce an example as in legam aliquid, puta carmen = literally "I'll read something, think/imagine a poem", i.e. "I'll read something, say, a poem" or "... for instance, a poem". Literally, that's basically telling your addressee to think, as an example for the time being, that the thing you'll read will be a poem.
Ah, interesting; I wasn't aware of that idiom, but it makes sense.