Q: What is this for? I have this kit and the instructions are very vague about this part that comes with it. I think it says it's to use on round duct but how and why?
A: To use probes in ducts, a hole is drilled.
The barrel of a cone is screwed into the hole in the duct.
The probe slips thru the center part of the barrel.
The thumbscrew is used to secure the probe in the barrel for hands-free use.