(Do you) understand? (Is it) understood?

[hzmonte]

This is related to https://www.usingenglish.com/forum/threads/221727-understand-understood

Often on the TV, people say "Don't kill people again. Understood?" instead of "Don't kill people again. Understand?"
1. Why?
2. Does "Understand?" refer to "Do you understand?" and "Understood?" refer to "is it understood?" ?
3. Must the responder say "(I) understand." and "(It is) understood." respectively?
(I asked a native speaker, in Los Angeles, who said "understand?" should always be used.)

 

[Rover_KE]

I'd like to welcome you to the forum and answer your question, but that will have to wait until you tell us the truth about yourself in your profile.

 

[hzmonte]

I told the truth the first time but your web site reset everything after I hit submit and then asked me to start all over again. I was frustrated and just hit where the cursor happened to be. After all you asked too many questions.

 

[Rover_KE]

Thank you.

We'll be glad to answer your questions when you have amended your profile.