I'm not exactly in the most inspired mood so it's pretty dry, but it does explain what it is.
When making the MR, I couldn't add any image yet since I already added a new icon for the assistants and will have to go back to create a picture and then add it here to the MR.
There is no fancy behaviour on Shift or Alt or Ctrl, so no need to explain it. (It does have a behaviour on Shift on the first handle, but that's not really something extremely useful since you can't do it with the fourth handle...)
Depends on: graphics/krita!1343 (merged)