Basically its a secondary that changes your dispenser into a robot dog that follows you around (can't be upgraded). And when you pull out the secondary (a whistle) it gives you 2 modes to choose from:
1st mode will slowly heals you and give you ammo like a level 1-2 dispenser.
2nd mode will alert if a spy is nearby (doesn't show where they are just will tell you if one is nearby that is disguised or invisible
But it can be zapped which will disable it until the engineer wracks it with its wrench (it will take longer to take it off then a normal zapper) it can be destroyed (about the hp of a level 1 sentry), but can be rebuilt for 100-150 metal
of course it would mean doing path finding for every engineer that has this equipped in the server, so if this can't/won't be added that's ok cause this will probably cause alot of bugs and glitches
(this is mostly to help battle engineers)