Commitment is a phase that all successful proposals go through. You can only be "committed" to 3 proposals at at time.
Everything you do to support a proposal is voluntary, if you don't do something there are no penalties.
When you "Commit" to a proposal you are promising to write 10 questions and/or answers. From the time that you click the green "Commit!" button until the commitment is completed is where the commitment count (3) comes into play. Anytime up until Beta Launch you can retract your commitment.
After private beta launch, you can start writing your 10 questions and/or answers. You can not "un-commit" after beta launch. If you are heavily active in the first few weeks your commitment can be completed and not taking one of your 3 spaces, shortly after public beta launch.
Worse case, if you don't meet your 10 Q&A quota the commitment stops counting against your count of 3 about six months after public beta launch.