Light and Ryuk go to a department store where Light shops for supplies to construct a hiding place for the Death Note. Light reveals to Ryuk his new Death Note hiding place in a secret compartment under his drawer. Light inserts the drawer he made into the top right side of his desk.
The drawer Light hides his Death Note in has a key to secure it in place, intentionally left in its lock. Light keeps a fake, regular diary inside the desk to distract people from the false bottom and justify the use of a key.
The drawer has a hidden compartment which is used for hiding the Death Note and can only be safely accessed using a particular method.
To unlock this compartment, Light unscrews an ink reservoir from regular ballpoint pen on his desk and pushes it up into a small hole underneath the drawer, lifting up the fake bottom to reveal the Death Note.
Even if someone figures out that there is a fake base, the drawer was designed to prevent anyone from ever discovering the Death Note.
Once the ink reservoir is inserted, it acts as an insulator, preventing the flow of electricity around the circuit. When the fake base of the drawer is put back, a piece of rubber fills the same role.
If someone were to try and force the bottom of the drawer open, however, the circuit would connect, igniting the gasoline contained in the thin plastic casing and instantly setting the Death Note on fire. This would destroy all the evidence that Light ever possessed a killer notebook. Upon potential questioning Light plans to say that he was hiding his real, personal diary in the drawer and destroyed it so it could never be read by anyone other than himself.