what does it do? it shows you what's being painted by first filling in the about-to-be-painted areas with yellow:
move your mouse around, interact with things and you get a good idea of where painting is happening, such as here where i was mousing over a toolbar button:
since painting is expensive and something that really impacts the experience of using software (flicker, latency, etc) this can be very useful to see just how well your qt and kde apps deal with painting, particularly if you have custom widgets.
best part is that it doesn't require any recompiling or fancy footwork: just export QT_FLUSH_PAINT=1 and start the app from that environment... for the above shots i simply went to a konsole tab and did `QT_FLUSH_PAINT=1 assistant`.
(and speaking of enjoy, the season finale of battlestar galactica kicked ass. so much so that i'm really not happy i have to wait until the fall to get my next fix ;)