Thumb king can either be played as an independent game, or be part of some other game or activity. Typically, it is played alongside some other activity "in the background", interrupting the main activity from time to time.
You don't need any equipment to play that game.
On player is chosen to be the thumb king. After some time - another activity can take place in the meantime - he puts his thumb on the table. The thumb should be clearly visible then, but the movement should NOT be eye-catching (or loud, or in any other way obvious) so everybody notices instantly. As soon as he does that, all the other players have to do the same. The last player to put his finger on the table loses. He has to do something for the group, or he gets a negative point, or whatever fits to the current situation.
After some time, another person is appointed to be the thumb king.
Combining with other games
This game mixes quite well with other games. For instance, it can be used during the card game UNO; each player who loses a round has to take two additional card. Thumb king is always the player who won the last round of UNO, or - more tricky - who last put down a "+4" card.