Being john malkovich what is used to enter the brain of malkovich?

In the movie "Being John Malkovich," a portal into actor John Malkovich's brain is accessed through a secret passageway located on the 7½ floor of an office building. This passageway is concealed behind a filing cabinet and leads to a mysterious tunnel. Upon crawling through the tunnel, a person becomes mentally connected to Malkovich and temporarily inhabits his body.