It's agnostic, the QUERY verb has nothing to do with the actual implementation, or the content encoding. You can use any content encoding for your query body, and you can resolve it any way you see fit.

In mine, I was just caching raw sql queries, so it was literally just text/sql encoding, the query in the body, some metadata in headers, and a sqlalchemy engine to execute the query.

It's basically a way to get around the non-idempotency of POST and URI limitations of GET.