This article is from the Apple ][ Emulator Resources Guide, by Alex Maddison with numerous contributions by others.
"Shrunk" disk-images were not originally intended for use with emulators, being a type of NuFX archive created by ShrinkIt. As opposed to a standard .SHK file which usually contains files, an .SDK file contains a whole compressed disk (for archival or transfer purposes). Unlike the similar Dalton Disk Disintegrator (DDD) archive, SDK images cannot be used directly with an emulator. The SDK image can be extracted to a disk-image under MSDOS or Unix using "Nulib" by Andy McFadden <email@example.com>.
PROGRAM LOCATIONS: http://www.jurai.org/~funaho/emulators/XGS/nulib324.zip (PC compile version 3.24) ftp://ftp.apple.asimov.net/pub/apple_II/utility/nulib.zip (PC compile version 3.03) ftp://apple2.caltech.edu/pub/apple2/ARCHIVERS/nulib324.tar.Z (Unix source) ftp://ftp.apple.asimov.net/pub/apple_II/utility/shrinkit34.sdk