It does a quick shake to dislodge any dust particles on the sensor, I always hold the camera face down during the clean, but sometimes it doesn't remove the bits that get stuck to the sensor (just the loose stuff).
There's a few ways of cleaning the sensor more thoroughly, I have a LensPen SensorKlear kit with a loupe to make sure I can see it's clean. There's also Arctic Butterfly kits which is a brush that spins to create enough static to attract the dust as you pass it over the sensor. Also there are various 'wet' cleans where you put a few drops of fluid onto a pad and wipe it on the sensor. Have a google and you will find a lot of info on all the types available.
Set your camera to somewhere about f22 and take a shot of a clear blue sky or a plain white surface without focussing and take a good look at the image produced. If you see blobs on the image then it'll probably need a better clean than the in camera system. Most of the time you wont see anything up to f8-f11 where most shots are taken (not landscapes though) so it's probably not worth worrying too much, unless you can see it on your photos.
