A wise old examiner once gave me the following explanation:
RETREATING BRAIN STALL
(ri-trēt'ing brān stôl): This is a condition of severe mental dysfunction, where the information flow in the advancing half of the brain (the input and retention half) is drastically unbalanced by the requirement for rapid output of information from the retreating half of the brain (the output half), resulting in a temporary stall of information flow from ANY part of the brain.
This debilitative condition is often found in student helicopter pilots, and occasionally in rated pilots, when they are under the stress of situations such as rigorous training sessions, phase checks, and especially, FAA check rides. This condition is transitory in nature and if encountered on the ground (such as during oral exams), usually clears up after a brief moment of relaxation. However, if this condition occurs in flight while one is acting as pilot-in-command, prolonged retreating brain stall may have fatal results.