Kuikae

Kuikae 「喰い替え」 is a rule applied to chii calls. It is commonly referred as "swap calling". When a player possesses a completed sequential group, like 4-5-6, then a player has the option of calling on a discarded 7. Upon doing so, it is commonly forbidden then to discard the 4, until the next tile draw.

Example:

1. 
2. Left player discards:
3. Chii is called to form
4. is left over, but cannot be discarded on the same turn.