I'm quite surprised the other military or ex-military pilots on here who have commented don't know the real background behind this reasoning.
There was a single pilot military aircraft many moons ago (I can't for the life of me remember the aicraft type) where the location of the gear lever caused a head turning movement as the pilot leaned over and looked for it and then lifted the head back up (the selector may have been on the floor, not sure...). This head twisting and leaning while the aircraft was turning was causing extreme disorientation and aircraft at night were rolling over on their back during the base turn.
I think it is called a Coriolis illusion but can't be sure, and remember it being demonstrated to me on a spinning chair. It is not like most illusions - it is a particularly violent, uncomfortable and sudden feeling.
The fix was to mandate gear extension during level, unaccelerated flight. Problem solved.
Today, where ergonomics play a much larger role in aircraft design this problem has hopefully been eliminated. The gear lever is generally installed in a location which requires minimal head movement, and multi crew also removes the issue as the handling pilot is not operating gear/flaps etc.
Moral of the story is... If your AFM says you can't do it in your aircraft type, don't do it. If you are single pilot at night, minimise head twisting/turning/lifting movements and consider gear extension in level flight to prevent coriolis illusion. It is nothing like the leans or somatogravic... just feels like your head has been hit by a bus.