What causes a UIViewController to become active?
<p>I am sure this is an easy question, but one that has escaped me for some time now.</p>

<p>Say I have a UIViewController, either defined as a root in an XIB or on a stack.  At some point in my code I want to replace it with another view controller.  Just flat out replace it.  How would I do that?</p>

<p>I have tried defining the controller and assigning, but not sure what actually makes it push on the screen with the absence of a navigation controller.</p>
