composite and smooth moving (double buffering?)

[Anders Storsveen]

When I enable composite and try to drag the konqueror window around
fast, the window will move very smooth, although very slow to, but
that's another issue (probably nvidia's rendr acceleration that is slow
right?).

However, this smooth moving, which only happens to qt apps, why is this?
Could someone please explain this to me, also how it works :)