The new trick is basically future-proofing -- something we never thought might be needed.
Now the bridge operates in upstart as well as the never-overwritten fixup script, and most importantly it allows you to sideload an emergency.sh script to the userstore, which will be run as root by upstart itself. To top it off, we have a functional su.
Amazon cannot play tricks with changing the EUID anymore.
It is still possible they could wipe the fixup script,
which was their own hijacked feature, so not nearly as likely...
In any event, see
Kindle_Touch_Hacking#Blocking_Official_Updates