http://ttevhjjsjxz6uqqcjkbig5cycd7n7xv7cmd6f5fcvrqaaa7f3bj36wad.onion/technology/security/pow/index.html
If it's
not enabled, the extension SHOULD BE ignored. If enabled, even if the suggested
effort is currently zero, the service follows the procedure detailed in this
section. If the service requires the PoW extension but received an INTRODUCE1 cell
without any embedded proof-of-work, the service SHOULD consider this cell as a
zero-effort introduction for the purposes of the priority queue.