Stance Change allows Aegislash to switch between Shield Forme (high defensive stats, low offensive stats) and Blade Forme (high offensive stats, low defensive stats). Aegislash is always in Shield Forme if it is not battling, and as such, switching out reverts it to Shield Forme.
If Shield Forme Aegislash uses a damaging move, it changes to Blade Forme immediately before using the move. If Blade Forme Aegislash uses King's Shield, it changes to Shield Forme immediately before using the move. Status moves other than King's Shield will not change its Forme. Moves that call other moves, such as Sleep Talk, will not change its Forme, even if they call a damaging move or King's Shield.
The Ability activates prior to and independently of any checks for status conditions that may stop the execution of the Aegislash's move, such as sleep or confusion.