You may pass under yellow if you are waived by, as in when a car is having mechanical trouble and is off line waving people by. I had a situation where I was in the heat of battle in the midst of a pass when I saw the yellow. The car being passed slowed for the yellow and I went right by. I then slowed, went off line and waved the guy past me. After the race I was given a "talking to" but it was noted that I had waved the competator past me, thereby not gaining an advantage. They seemed satisfied with my handling of the situation.
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I've been involved in three of these in the past couple of years where the guilty PUY party waved the other car back by while still under yellow, and each time 1.) the original PUY was not seen by the corner worker 2.) the re-pass was seen and was reported as a PUY 3.) The PUY and re-pass with wave-by were explained to the steward in impound. Each time both drivers were released without incident.