I've subscribed to Four Wheeler Magazine for a number of years and I've wheeled since age 16 and here's the take that I've gathered:
Rock crawling is the foremost example of when tires should be aired down. Airing down results in a larger footprint and easier adapatation of the tires (i.e. allowing sidewall flex). The tires are able to "grab" the rocks and create an envelope on HARD surfaces.
Trails that have large obstacles or jagged rocks may also warrant airing down, with the same premise as above.
Sand appears to be an open debate. If the sand is deep enough, then it is fair to say that partial airing down may be of benefit. Airing down may help with keeping atop the sand, versus driving the front wheels downward.
Mud bogging, Snow, and light-duty trails do not require lowering the psi.
I've only aired down a couple times in the past. I chose to do so when coming to sections of a trail that had medium size rocks and I was more concerened with punctures than traction.
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