It's simple, a tube is connected to the Reimu in what would be their uterus or whatever and it pumps sleeping baby yukkuris on it, the mother still think she's giving birth constantly and expells the babies, a continous cycle of forced birth.
Maybe it's a method of both torture and to make premature yukkuris that feel from their stalks or come out too early, to have a "2nd chance" to from better, the tube may be filled with some liquid that keeps them healthy and alive until they finally are inside the Reimu, which starts the process of development making them overcome their premature status and be full fledged yukkuris.