Off screen rendering mode in iOS

In CALayer class, there's a property named shouldRasterize. In documentation, it say that when this property is set to YES, the layer is rendered as a bitmap.In WWDC 2010 videos, session 123: Building Animation Driven Interfaces, it say wh开发者_如何学Pythonen this property is set to YES, it renders off-screen and cache a view and all of its subviews.

I don't understand about off screen rendering mode. Please explain for me. Thank you!