“They must've had a Fluctuator alter the stones' density. You'll understand once you feel them.”
A Fluctuator is a special ability or possibly a gadget that allows an elf to modify the density of something. The Neverseen have a fluctuator in their ranks. This ability/gadget is useful for hiding secret hideouts or anything else without a door. This ability/gadget was mentioned in Book 5: Lodestar, when Mr. Forkle takes Sophie and her friends to the hideout that the Neverseen took Sophie and Dex to in Book 1: Keeper of the Lost Cities. Tam pushes his body through rocks because the Neverseen used a Fluctuator to change the density of the rocks. Also, a Fluctuator probably altered the density of a wall in Quinlin and Livvy's house, leading to his secret library.
However, it is unclear whether Fluctuator is an ability or a gadget, for it was only mentioned once.