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the kernel via a magic ioctl (see shd/shd.h).  For example:

	imagedump -C /dev/shdXX /usr/testbed/images/FBSD410-STD.ndz

Would build a list of allocated ranges for the given image and pass them
to /dev/shdXX via an ioctl.

The checkpoint driver can return a list of blocks that have been modified
since its startup.  If you use "imagezip -C /dev/shd0 -s 1 /dev/ad0 foo.ndz"
imagezip will make an ioctl call on the shd device to get the modified
blocks it needs to save (rather than parsing the filesystem).  It then
creates an image using those blocks.

The end result should be an "incremental" image that can be put down on
the disk by frisbee after the original image has been loaded.

This is all a theory right now (including the ioctl :-) but should get
tested RSN.