Is there a performance boost by using yield with foreach?

After reading a few articles : http://csharpindepth.com/Articles/Chapter6/IteratorBlockImplementation.aspx http://msdn.microsoft.com/en-us/library/9k7k7cf0.aspx

There is one thing that I didn't quite understand and was curious about.

I was curious to know a foreach loop can utilize yielded function's results before all the yields have been completed.

Take the follow functions for example:

    public static IEnumerable<int> RandomFunction()
    {
        for (int i = 0; i < int.MaxValue; i++)
            yield return i; 
    }

    public static void PrintRandomResult1()
    {
        foreach (var i in RandomFunction())
            Console.WriteLine(i);
    }

    public static void PrintRandomResult2()
    {
        IEnumerable<int> Enumerable = RandomFunction();
        foreach(var i in Enumerable)
            Console.WriteLine(i);
    }

If we use PrintRandomResult2 I would assume the Randomfunction would have reached all of its yields before the foreach beings; however, if we call the RandomFunction directly in the foreach will the iterations begin before RandomFunction is complete?