Here is an interesting picture. I will take better ones later today. Grab a coffee.
If you zoom in on the circles you will see fan leaves growing at 90 degree right angles to the light.
What they are actually doing is putting their backs to the wind.
I run my VPD on the higher end of safe, because I know my setup really well and it's not as dangerous as it can be if you aren't aware of whats going on.
Many would say light avoidance, but what is really happening is this. VPD is all about transpiration, which is evaporation.
Wind accelerates evaporation. It's my green fan.
So when I set a high VPD and up the rates of evaporation (transpiration to be correct) the plant is walking a fine line, so when the fan adds evaporation, the plants will turn a leaf or two to slow evaporation, thus fine tuning their own VPD. They lower the surface area of evaporation by turning sideways and blocking wind.
The moral of the story.... If you have a few leaves turning away from your fan, and it's not in hurricane mode, you are likely very close to over-revving your plants.
Don't turn your lights up any more. Check your VPD. Be careful. Maybe turn the fan down if you are worried, but don't turn it off.
But.... do it successfully and get more out of your plants.
It's an indicator to watch for. I strive to reach it, but if you are still a beginner, look at it as a warning and slow down just a little bit.
Don't go there just because Gee showed it to you, spend an afternoon researching the basics of VPD to grasp it, and it's a simple concept once you read a bit, and then if you want to push your plants, at least you know where danger lies.
VPD is the throttle of a plant. The plant doesn't push water from the soil and through itself to exit the leaves, the atmosphere sucks on the plant like a straw and pulls the water through. If you suck too hard the water goes through to quickly and either can't load up with nutes enough, or goes through too fast and the nutes can't be used in time. Either way, it shows as a deficiency.
It's called "rate of transpiration".
Many nute deficiencies are actually a much to high VPD, over-revving a plant to the point that it is photosynthesizing faster than it can supply nutes from the rootball, which will cause a deficiency to occur even though the soil is good.
Thats a good one to bookmark for future troubleshooting. If you have a deficiency, the 1st thing you should check is your VPD. It may not be a deficiency at all.