This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
At line 3, you are telling x to refer an object of Y at runtime. So, you can "get the object back" from x at line 4. But if you remove line 3, then x is pointing to an Object of X (which has no knowledge about Y). So obviously you cant ask x something which it doesnt know about. Hope this helps.