For instance, a 45v45war. Clan A has 50 members, clan B has 50.
If a member in the top 45 leaves clan during prep day, then the person ranked 46 earlier would then enter the war.
Sounds fine i guess, but any member who leaves during war is a total jerk. A good policy is for leader to ask who will war several days before, and kick those who can't/ won't participate right before he/ she starts war, then reinvite them after he/she starts the clan war.