A Spying Portal is a weave in the One Power. It is very similar to the portal for Traveling, but it opens in horizontal, not in vertical. It was conceived by Yukiri opening it very far above in sky to spy an enemy army on the ground.
Yukiri made also two modification to this kind of portal: added a weave to make it invisible from the enemies on the ground and another one, like a transparent not frangible glass, to prevent anyone to drop into the portal by accident.