It's basically a variation of HyDE (Hypothetical Document Embeddings), and the rationale is that the embedding of the query is not necessarily close to the embedding of the answer. If you generate a hallucinated answer, it can line up with the actual document better (in the embedding space, via BM25, or hybrid).
But honestly, it only works for common knowledge that's already in the LLM. If the target document contains very niche or private information, then the hallucinated answer's embedding can be even farther away than the query's.
I feel like querying for the smaller distance from the set of embeddings of both the query and a fake answer structure might solve that. If there's something very close to the query, it dominates, otherwise the fake answer is the guide. Plus, why not use an LLM to judge between them if we've got a token-burner all warmed up to start.
But honestly, it only works for common knowledge that's already in the LLM. If the target document contains very niche or private information, then the hallucinated answer's embedding can be even farther away than the query's.