Sure I can... The PatternIndex is usefull when you are using a tiled bitmap, with that I mean, say you have an animation of a walking mario, and say each your total picture is 690x64 and each frame of your animation is 64x64 then you can set the patternwidth of the image to 64 and the patternheight to 64. Each time you draw the image to the screen, a rect is calculated with the size of your frame, so in short it only draws the frame you want, so if I wanted the 4th frame I set PatternIndex to 4, if I want the 10th frame then I set the patternIndex to 10...

I hope I make a bit of sense... It's hard to explain without having examples... Take a look at the pacman image over at this URL
http://delphigamedev.name2host.com/dxtut3b.htm and you'll see what I mean, each pacman frame has the same with and so you could make an animation using the patternindex (the tutorial uses the SpriteEngine to make animations, but if you want you can do it yourself with the patternindex).