At the start of your Command phase, in a turn in which you have not called a Waaagh!, if you have one or more WARBOSS models on the battlefield (or embarked within a TRANSPORT that is on the battlefield), you can call a Waaagh! for a second time this battle. When doing so, that second Waaagh! only counts as having been called for WARBOSS, NOBZ and MEGANOBZ units from your army.