When you abdicate, if your Allegiance has not abdicated this Seat of Power in the last 72 hours, you will receive an Allegiance Shield. During this window, another member of your Allegiance can take the seat by tapping and selecting take over. No one outside of your Allegiance will be able to attack or take the Seat while this shield is up, but you will need to work quickly since it will only last for 3 minutes.
Remember, you will not receive an Allegiance Shield if you have abdicated in the last 72 hours. You will receive a pop-up warning if no shield will be applied, so you can choose to halt the abdication and wait or continue without the shield. If no one in your Allegiance takes it after 3 minutes, the shield will fall and the seat will return to NPC control.