One and Done is intended to disallow players with invitations and travel awards to the World Championships to play in another qualifying event (Regionals, Gyms, States, Grinder). States is a qualifying event for trip awards for Nationals, however Nationals is open to all.
For example if a player qualifies for Worlds 2006 at a Regionals, they will not be able to play in any Gym Challenge or the Grinder for the rest of that season. If a player wins a Gym Challenge then they will not be able to participate in the Grinder for that season, although this was the previous policy for all of the other season's Grinders.