http://ttevhjjsjxz6uqqcjkbig5cycd7n7xv7cmd6f5fcvrqaaa7f3bj36wad.onion/technology/security/pow/index.html
In summary, the Proof of Work (PoW) defense is a mechanism that imposes a
disadvantage for those who make a huge number of connection attempts to an
Onion Service, with a small side effect of requiring other users to expend a
little more effort when trying to connect if the service is under high load.
With the PoW defense, attackers effectively work against themselves: the more
attackers hit the service, the harder it gets to hit the service.