What is the `unsafe` option?

What is the `unsafe` option and when should it be used?

I looked through the source code and `unsafe` doesn't seem to make a difference. `req.flash` in the following is always `undefined` every time the middleware is called on every Http request, so that test condition will always be `false`.

function(req, res, next) {
if (req.flash && safe) { return next(); }
req.flash = _flash;
next();
}

Start at the middleware code containing the shown req.flash assignment and trace how the unsafe option is handled. Clarify what the option changes, when it should be used, and document the behavior and example usage in the project’s documentation.

Written by the indexing model from the issue text.