How does Express/Connect middleware work?

I am learning Node.js, and I have read some tutorials, like The Node Beginner Book for learning the core funcionality. But the more I read some examples, the more doubts I start collecting.

On the further example, obtained from a tutorial, we can see that for a CRUD 'read' request for key /documents/titles.json, we are returning a value:

app.get('/documents/titles.json', loadUser, function(req, res) {
  Document.find({ user_id: req.currentUser.id },[], { sort: ['title', 'descending'] },
  function(err, documents) {
     res.send(documents.map(function(d) {
        return { title: d.title, id: d._id };
     }));
  });
});

On this example, the function loaduser() is used for authentication purposes:

function loadUser(req, res, next) {
   if (req.session.user_id) {
      User.findById(req.session.user_id, function(err, user) {
         if (user) {
            req.currentUser = user;
            next();
         } else {
            res.redirect('/sessions/new');
         }
      });
   }
}

What I don't understand is:

1. I suppose that node.js, before start executing the app.get, it goes to loaduser function. loadUser() function has three parameters: req,res,next, but I don't see, at least, how you pass from app.get() the "req" parameter to loadUser(). From where does it come?
2. Inside loadUser() function, when you execute next(), it means that the function app.get()" can continue its procedure, but this req.currentUser = user, is the same req that is used on app.get() function?
3. Inside loadUser() function, when you execute res.redirect() code, automatically breaks the procedure on app.get() function, right? it looks like it doesn't return to Document.find().