Curious about the exact function of the bottom infrared sensors.

[Yorkshire_Pud]

I wrote DJI and this is the official response from them

"Hi there. Thank you for bringing this inquiry to us. The Infrared sensor can detect altitude, so it will use with the downward vision system. Also, these sensors use infrared technology to detect obstacles and avoid collisions, making the drone safer and easier to fly. Hope this helps. Thank you for your valued support."

I think the "avoid collisions" bit is badly worded.
VPS can, to some extent, prevent the drone from landing on something untoward but it can not detect high spots that the drone is approaching.
Where my drones pass over a 'high' object, that is close to the bottom of the drone, the Landing Protection does kick in and lift the drone but it's always a delayed reaction.

 

[Walleye Hunter]

This is an interesting thread, thanks to all for the information.