how to show part of big image on mouse rollover

I want when user roll over on my image it shows a part of big image of the same picture i known its possible in flash but how i don't Known ?? here is the link of the live example the开发者_开发技巧 site use the same technique in flash click here

Thank you in advance

the big image is masked with the magnifier sprite and it's moved according to the position of magnifier (to show the correct part )