Referencing objects using ´this´ from inside a function, concept not well understande

Friends help me

I understand this, _parent keywords and its usage but I don’t understand the following:

MovieClip.prototype.move = function() {

function toRight() {
this._x=50;
trace(this._name);
}
toRight();

};

Then, I use the move function the onRollOver event on certain movieClips. The tracer output is ‘undefined’ and the object doesn’t move when I put the mouse over. Friends, tell me why the ‘this’ inside the function doesn’t reference to the object which the code is applying? If I put this._x=50; outside the function toRight() it’s ok. Please, don’t tell workarounds to get the same effect, I want to know why ‘this’ lose its reference inside the function toRight(). I am learning in Flash Professional 8.0:trout: