Understanding highlight modes

I have been trying to find information about how the highlight (recovery) modes work, but haven't found much (btw, does this forum really have no search feature?)

In particular I wonder what does the 'blend' highlight mode do? What is being blended?
I looked at the implementation, but it is a bit too cryptic for me to figure out what is happening.

And what is the difference between 'unclip' and the different 'rebuild' variants?