How to flip button in iphone sdk?

I would like to know how buttons are flipped in the following app:

http://itunes.apple.com/app/brain-cafe-geoquiz/id334815548?mt=开发者_Python百科8

I am refering to this screen in the app:

Refer the following code:

CGContextRef context = UIGraphicsGetCurrentContext();
context = UIGraphicsGetCurrentContext();
[UIView beginAnimations:nil context:context];
[UIView setAnimationDuration:0.75];
[UIView setAnimationTransition:UIViewAnimationTransitionFlipFromRight forView:button cache:YES];
[UIView commitAnimations];

You can mimic inner shadows in CoreGraphics... see Inner Shadows