How do backup apps which create a system image handle disk changes during the image creation process?

Question

I created a backup disk image of my disk yesterday and the software told me to close all Windows programs to make sure the process finishes successfully.

I did that, but I was wondering what happens when some program does write to the disk nevertheless during the process. Windows 7 is a complex system and surely various log files and such are written continuously (the disk has one partition which contains the Windows install too). How does the backup software handle it when the disk content is changed during image creation?

What is the algorithm in this case?

Answer 1

Snapshotting, or 'Shadow Copy' as Microsoft calls it, see Shadow Copy on wikipedia