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From The Apple Wiki

This was used to get root access. The following command has been used in the application that initially gets installed when applying the evasi0n jailbreak:

#!/bin/launchctl submit -l remount -o /var/mobile/Media/mount.stdout -e /var/mobile/Media/mount.stderr -- /sbin/mount -v -t hfs -o rw /dev/disk0s1s1

This is the only content of this app. The above file contents tell the kernel to execute launchctl with those specific arguments.