The idea is some sort of “stamp” brush. The brush can “pick” some area and use it as brush tip. Similar to clone brush or smudge brush in smearing mode but there is bunch of differences:
1) Unlike clone tool, it generates brush tip only once
2) because of p1 you can rotate, scale, mirror, change ratio and add texture or scatter effect
3) ability to create brush tips from another file (can be very useful in some cases)

Some use cases:
1) Creating a mass of similar objects (grass, leaves, rocks, flock of birds, bullet holes, etc, etc)
2) Creating texture (especially with combination of different blending modes)
3) Creating neat brushwork

There is cool video that show some use cases (and how it works):
https://www.youtube.com/watch?v=Vcz5Fh6Wcx4
Also I experimented with creating brush tips via clone brush and got some examples as well:
http://i.imgur.com/PW8j2rl.jpg

You can try it yourself (sort of):
1) Choose clone brush
2) Set brush tip and size\rotation
3) Select source area and click in some clean area
4) Create a brush tip from this stamp (don't forget to delete it later)

Possible further improvements:
1) Workable HSV settings (currently HSV work only if “use color as mask” checked).
2) Ability to paint on the created brush tip itself (use different colors, presets, blend modes, even filter brushes)
3) Easy way to create animated brush tips (?)

Actually it’s pretty simple engine, just pixel brush but with mechanism of making temporally brush tips from picture. I’m not a developer but I think it easy to implement. Probably creating the whole brush engine isn’t necessary for core feature. But it would be hard to do further improvements.

Possible problem - animated brush tips and stacked brushes later. Possible solution – somehow creating all tips at once. Or, copy source area bigger than brush tip and create tips from that (but scale, rotate and mirror it accordingly).

You mean creating a brush tip from the clipboard? because as far as I know, that is available, and you can even preserve the colors if you uncheck use color as mask http://i.imgur.com/4w8q6Z4.png

Yes, you can create brush tip from clipboard but it required a bunch of actions for each one (+ deleting throwaway tips).
The main feature is ability to easy create tips on the fly and automatically delete them after usage.