There is a button or lever located on the outside of the mounting ring, on the shutter release button side, lower quadrant of the ring. Mounted lenses, AF or AI do not make contact with this lever or button, but it is depressible and is spring loaded. I cannot see that is makes a difference when depressed manually. What is it for? It is undocumented in Nikon manuals. I found it by accident and at first thought I had broken a piece of plastic and it was jammed in the lens mount, but not the case.
On non-G type AF lenses and AIS lenses (lenses with an aperture ring), there is a tab on the aperture ring that will engage with the tab you are describing when the aperture is set to its minimum (largest f-number). Without that, Auto, Program and Shutter-priority won't work properly (and you will see an error instead of an aperture value on the display in Manual).