This link goes to a fascinating little explainer by Popehat on Twitter. Excerpted here and lightly edited for readability:
The Fifth Amendment says you can’t be “compelled in any criminal case to be a witness” against yourself. The government can’t compel testimony from you if it might incriminate you — at least not without a grant of immunity.
One of the issues the courts have had to confront is this question: what is testimony? Is a sobriety test testimonial, especially if you have to talk? A blood test? Mandatory filing of tax returns? A handwriting sample? And so forth.
Now, pre-existing items and documents are not testimony. It’s not testimonial if you’re forced to turn them over — in the abstract. But there’s a nuanced, complicated exception. Sometimes the “act of production” can be testimonial.
The idea is that if the government subpoenas you for documents, and you turn them over, you are admitting the existence, authenticity, and your possession of the documents — and those may be incriminating. So even if the documents aren’t testimony, producing them might be.
So years ago, the Supreme Court decided that sometimes, if it wants to compel production of documents, the government will have to offer what’s called “act of production immunity” — they can’t use your production of the documents against you.
Then, in 2000, the Supreme Court expanded the doctrine in the case of Clinton associate Webster Hubbell. The case involved very broad fishing-expedition style subpoenas from Independent Counsel Ken Starr.
The subpoenas were very broad — they asked for things like “any and all documents reflecting, referring, or relating to any direct or indirect sources of money or other things of value received by or provided to” Hubbell.
The Court found that because of how the questions were framed, identifying documents in response to them was “the functional equivalent of the preparation of an answer to either a detailed written interrogatory or a series of oral questions at a discovery deposition.”
Put another way, rather than simply physically turning over documents, Hubbell had to use “the contents of his own mind” to select documents responsive to the government’s broad fishing questions. That made his response testimonial, and thus protected by the Fifth Amendment.