Git hooks and stdin/stderr/stdout.

Dear all,

I have a question regarding git hooks and would have liked to get some
better understanding of how internally a hook is run. Especially I
want to know  what happens under the hood because the behavior kind of
puzzles me. So what I've understood:

When git runs the hook, git does
- connects stdin of the hook process to it (is that always the case?)
- redirects stdout of  the hook process to the stderr of the hook
process. So anything you write to stdout/stderr in the hook process
ends up in the stderr of the git process.

Is that understanding correct? Is there a doc describing this?

Thanks for the help.

BR Gabriel