Recognition between two souls sparks a new soul that waits in the beyond until recognition is fulfilled and a new body is made. Then the new soul goes into the baby. I get it from Winnowill's words to Rayek in Siege at blue mountain : "this isn't the joining of two souls to make a third". The waiting in the beyond part comes from the story about the Tricksters clan in Botc 5, where Zarhan fights against recognition to Cheseri for ten years. both parents could feel her screaming in the beyond with anger and frustrasion while she waited.