What triggers in-year offers from waiting lists?

[smogwod]

My DD is 2nd on the waiting list for a school, to go into y2 in September. I've heard, from the school, that they know of 2 planned leavers - yay! One has already been offered a place at another local school and another is moving out of area. Does anyone know at what point the LA are able to offer those places to the waiting list please?

spoke to head of one school where she is 2nd on the list .. but she said she is not aware of anyone leaving so it would be better if we visit the school once a place becomes available...