Chemical Concept Demonstrated: The reduction of silver ions to silver metal
The entire inside surface of the flask is coated in silver metal.
The inside surface is coated with a silver amine [Ag(NH3)2+] in the first step. This compound can be reduced to form silver. Although the reducer (possibly dextrose, but almost certainly a similar hydrocarbon) wouldn't be able to do this normally, the activator (essentially a strong base) makes the environment suitable for such a reduction. Because the ions of the silver solution and the reducer only touch each other on the inside surface of the flask, the inside surface is the only place where silver metal forms.