We don't publish any technical details -- if you have some internal/additional questions, you can write me email: kurtin@avast.com
Our sandbox have some special settings for common web browser (IE/FF/Chrome/Opera) and you can set if history/cookies/... will be saved for next web browser run, or deleted. When you download something from web browser, it should be detected (as standard Save-As operation) and excluded automatically from the sandbox (if you think it's not working, let me know your OS + web browser version, I'll try to repro it here).
>> Are programs given full access to the machine even though they're sandboxed?
Sandboxed apps, even with maximum access rights, will be automatically blocked from destructive operations (i.e. all their changes will remain in the sandbox; once they're terminated, their changes will be destroyed). Those changes are visible only from view of the sandbox.
>> sandboxie comparison
Some features are implemented better in sandboxie, some of them in avast. I tried to add features to support web browsers, we also use hardware virtualization CPU feature (VT-x/AMD-v) for sandbox on 64-bit OSes (sandboxie does not) and I think this is very significant difference.
If you need more technical details, write me email; if you need add a feature to sandbox, let me know as well. Thanks.