Full Idea
If a brain implant would compel you to perform an action which you in fact freely choose, then you are free, but couldn't have done otherwise.
Gist of Idea
You can be free even though force would have prevented you doing otherwise
Source
report of Daniel C. Dennett (Elbow Room: varieties of free will [1984], §6.1) by PG - Db (ideas)
Book Reference
Dennett,Daniel C.: 'Elbow Room - Free will worth wanting' [MIT 1999], p.132