When a metzora came to the Beis Hamikdash at the end of his period of tumah, one step in his purification procedure was to immerse in the mikveh located in the Chamber of the Metzoraim.
The metzora would then stand in the Nikanor Gate where some of the blood from his asham offering (and some olive oil) was applied to his right ear, thumb, and big toe. The metzora is not permitted to enter the Azarah until he is completely tahor, but the blood would be disqualified if removed from the Azarah, so to satisfy both restrictions the metzora would remain outside the Azarah but lean in to the Azarah for the blood applications to take place. As a general rule, the space taken up by the Azarah walls (which were five amos thick) was imbued with the sanctity of the Azarah. This meant that as soon as a person standing on Har Habayis walked beyond the outer edge of the wall and entered the space within one of the Azarah gates they were considered to be “in” the Azarah. At the Nikanor Gate, however, the Sages ruled that the Azarah sanctity only began at the inner edge of the walls (see diagram). This allowed the metzora to shelter beneath the lintel of the Nikanor gateway while his blood and oil applications were carried out.