The Golden rule is that once you lock you are tied to the lock for however length of days you locked it for. However, if the rates drop by .25% or better on your file and its the same credit or cost as your original lock, the investor will allow you to take advantage. They want to keep your business and hope you don't go to another investor.
Be careful of all the scam out there preaching you can float your rate down whenever you want, that's not true. There is a minimum requirement and rates do have to drop by .25% in order for you to take advantage of a free float down.