What Geminus amounts to is an old Amiga feature called Smart Refresh - where if you overdraw something with a window, the OS takes a snapshot of the window underneath and when you drag away, it restores it. It takes all the traditional "send a signal to the app to repaint" methodologies away.
Geminus is just clever bitmap caching and redraw.. however with a UMA graphics system (i.e. embedded in the chip, uses the DDR2 RAM) there's very little benefit in caching bitmaps in "graphics" memory.
FYI the graphics core in the iMX515 is practically identical in feature set to the one in the XBox 360 - just running at a lower clock rate.