From Episode 2 of the Death Note (DN) anime series, “Confrontation” “Taiketsu”, in English Dubbed.
Light Yagami shows Ryuk how he will stop anyone from ever touching the notebook. He then opens the drawer in his room to reveal an ordinary diary, saying that anyone snooping around would most likely see that diary and stop there. This diary is locked up inside the drawer. However, the key to the death note is a bit more complex. Light then removes the ink cartridge from a ball point pen and puts it through a hole at the bottom of the drawer to reveal a false bottom, where the Death Note will be hidden.
The cartridge acts as in insulator, preventing the flow of electricity around a circuit Light made surrounding the drawer’s inside. When the false bottom is in place, a rubber stub attached to the bottom of the false bottom acts as the insulator, instead. If anyone were to forcibly remove the false bottom from the drawer, the circuit inside the drawer would be completed, thus igniting the bag of gasoline the circuit is connected to and burning the death note. If anyone were to question Light about the burning of this notebook, Light comes to the conclusion that he would merely answer that that he didn’t want anyone to read his real diary. Ryuk compliments Light about how easily he was able to come up with a solution, but notes that Light must take the risk of burning himself when handling the notebook now. To this, Light coolly says that he would much prefer risking a house fire over being given the death sentence.