This has changed in general for iOS7 but for smooth transitions, check the iOS7 Migration guide, shown here below:
Navigation Transitions
On iOS 7, when transitioning between two windows in a navigation stack, the bottom window slides half way off the screen, pauses then the top window appears. This is noticeable if you do not specify a background color for your top window, which makes it transparent. As long as you specify a background color for the top window, the application will appear to transition the same as previous iOS versions.
On iOS 6 and earlier, the bottom window would slide off screen as the top window appears on screen at the same time.
So essentially just set the backgroundColor
for the top window.