[tor-relays] ExtORPort notice

Upon upgrading obfsproxy to 0.2.6 and Tor to 0.2.5.1-alpha-dev (git-f63b394d90583b77+96972c4) for scramblesuit, I got this in the Tor log:

Feb 15 04:40:03.000 [notice] We are a bridge with a pluggable transport proxy but the Extended ORPort is disabled. The Extended ORPort helps Tor communicate with the pluggable transport proxy. Please enable it using the ExtORPort torrc option.

How should this be set? What does it do? I saw some web pages suggesting "ExtORPort 6699" for statistics-gathering purposes.

Thanks, Delton