Atama mahjong 「アタマ麻雀」 is a modification to mahjong rules. It features the ability to call on tile by virtue of possession of a single tile. In other words, tile calls can be made to complete pairs or atama, hence the name "atama mahjong".
In terms of speed, game play heavily favors tile calling. The emphasis on pairs increases the usage of chiitoitsu.
- 【アタマ麻雀】のルール説明するよ！ (YouTube)
- Game tutorial on Atama rules.