AVAST Sandbox

I don't get the AVAST Sandbox. I have tried doing some searches but it is hard to wade thru the junk to find an explanation that is simple and informative.

I read something about it undoing any changes. So, does that mean if you run something in the sandbox, and it updates the register or a file, that will be undone. And then, if you really want the changes it does to be kept -you would have to run again outside the sandbox?
How would you know whether or not it was safe to run outside the sandbox - after you hae run it in the sandbox?

Fay

On the Avast forum, a guy had posted a tool to use to test the AVAST sandbox.
It wrote a small file to the root of the C: drive and put an entry in the registry.
If ran outside of the sandbox, after it finished you would be able to see the file and the registry entry. If ran inside the sandbox, you should not be able to find either after it finished.
(Well I had to run it as administrator for it to actually put the file in the root).
It worked- Avast opened and said a potentially harmful program was trying to run and asked if I wanted to run in the sandbox. If I said no - then it ran and I could find the file and the registry entry. I deleted both and ran again and said yes. It ran and I could not find the file or the registry entry.

But, I still have the question, so you run it in the sandbox. Then how do you know whether or not it is save to run the regular way>

Fay