The four types are based on the possible configurations regarding complete and incomplete tile groups.
Atamanai or headless is a shape, where the hand lacks a pair. Ideal shapes would involve four tiles forming two ryanmen each. If either of the ryanmen completes, then the hand becomes tenpai with a tanki. Otherwise, any of the four tiles can pair up.
With an ankou in the hand, the above shape becomes even more advantageous. The completion of either of the ryanmen can bring about pinfu by discarding one tile from the ankou and use the remaining two as the pair.
- 【麻雀】上がりを増やす！イーシャンテンの秘密【初心者向け講座】 (YouTube)
- Video covering iishanten shapes