When playing with overs, any of the players at the table may ask to have an "overs button". The dealer usually keeps a stack of these buttons in their tray, and will hand one out to any and all players who request them. These are usually bright plastic buttons with the word "overs" on them; the point of the button is that it must be easily seen by any player at the table.
If, at the start of any betting round for a hand, all remaining active players in the round have overs buttons, the stakes for that betting round are immediately set at the higher "overs" level. Often this is double the original stakes, but it may be any other level specified by the casino. If even a single player without an overs button is still active and able to bet at the start of a betting round, the stakes are not raised and remain at the original (normal) stakes.
Overs are a method of providing higher-limit games to a small selection of players who might not be able to populate an entire game of their own but don't mind playing slightly lower limits if that's all which is available. In this method, players who don't have overs buttons play the normal stakes at which they are playing and are never affected by the overs (unlike a kill, which raises the stakes for all players on some hands); they need not even care about other players' buttons for any reason. Players with overs buttons are playing at higher stakes whenever they are up against only other overs-button players.
While usually seen at the highest regular limits in a cardroom (e.g. having 20/40 with 30/60 overs) to allow for a few high-limit players, overs can be used in even small cardrooms to keep a game going between low-limit players and higher-limit players. For example, the Hotel Del Rio and Casino plays a 3/6 game with 6/12 overs so that their (usual) single game can run frequently.