How do I update an ISO with zsync?

Turns out I was being dense. A correct Googling and I found my answer on the Ubuntu Wiki. All I need to run is:

zsync http://cdimage.ubuntu.com/daily-live/current/oneiric-desktop-amd64.iso.zsync

I should point out that my previously-downloaded ISO is in the same path as this.

It downloaded the zsync file, checked it against my ISO and updated it to the latest version, 162MB spent. Bargain.

Testing daily builds with bandwidth caps, I started trying to figure out how to go about this. Oli's posted technique led a response of "No relevent local data found - I will be downloading the whole file."

This is what I had to do to tell zsync to look for the right iso to build off of.

zsync -i /path/to/old/trusty-desktop-amd64.iso  http://cdimage.ubuntu.com/ubuntu-gnome/daily-live/current/trusty-desktop-amd64.iso.zsync

I hope this helps someone else.