gistfile1.js

// When you define your module, you specify a function to run when it's being added in.
// This includes a `Y` object:
YUI.add('myModule', function(Y) {
   Y.Foo = function() { alert("Foo"); };
});

// When you *use* your module, you declare a similar function that gets a `Y` object passed in.
YUI().use('myModule', function(Y) {
   Y.Foo = function() { alert("Bar"); };
});

YUI().use('myModule', function(Y) {
   Y.Foo(); // Will alert Foo, since you didn't modify anything in this namespace
});