There are people that believe you should have no backbend in your handstand at all. That only the straight handstand is how to do a handstand.
But I don’t believe that. Just look at how many contortionists balance using excessive back bend.
When we’re talking about handstand backbend we can be talking about two things.
1) How much backbend you have in your handstand.
2) Going from a handstand into a backbend or a bridge. You might also call this a handstand to bridge.
This video shows you both.
If you’re trying to do this, you don’t need as much back bend as this fellow has but it certainly helps. I would highly recommend improving your gymnast bridge before trying this exercise out.