Handoff is a server side helper script that enables extra functionality to the off hand.
banners on map navigation particles
Holding a map that has marked banners in your off hand will emit particles that point to the banners in the world. Particle colours correspond to the banner colours.
coordinates in books navigation particles
Writing coordinates in a book and holding the book in your off hand will emit particles that point to those coordinates in the world. The format for the coordinates are <colour>/<X>/<Y>/<Z> or <colour>/<X>/<Z> (leaving out the Y coordinate).
Examples:
My cool mountain base
blue/456/200/-345
Farm
yellow/32000/-6000
specific map ID exchange
Renaming a blank map in an anvil to the ID you would like and placing it in your off hand will replace it with that map ID. This can be useful to gain access to maps that have been lost to time on papyri.