In the summer of 1942, the Nazis threatened to carry out house searches if the Dutch continued to refuse to hand in their bicycles. Bicycles were valuable assets in wartime, which is why the Nazis wanted them.
If the front of the building on the Prinsengracht was ever searched, the people in hiding would very likely be discovered.
It became therefore necessary to protect the hiding place better, and helper Johan Voskuijl built a revolving bookcase. This video shows how the secret entrance worked.