Fluctuators allow an elf to modify the density of an object.
The Neverseen used a Fluctuator, which Mr. Forkle deduced when they visited a hideout in Lodestar. The entrance to the hideout used rocks with altered density, allowing elves to pass through them. In Nightfall a fluctuator may have been used in Quinlin and Livvy's apartment to lead to a secret library.
In Unlocked it is not listed among the list of "the most well-known special abilities". However, it is also not listed in the section listing the "most well-known gadgets", leaving its status as either gadget or ability still unconfirmed.
Quotes[]
“They must've had a Fluctuator alter the stones' density. You'll understand once you feel them.”