AMD hasn't publicly talked about this new means of 2D acceleration or even privately explained what's all changed. Though from the sounds of it, it's basically like using ShadowFB with Catalyst. For the primary frame-buffer, it's using a shadow frame-buffer with the CPU.
Enabling this 2D improvement can be done by running aticonfig --set-pcs-u32=DDX,ShadowPrimary,1. Run at your own risk.