I have a hand driver set from iFixit. There's a part included that I can't identify, and because it's an older version of their set, isn't listed on the parts list for the product when I look it up now. In the photos below it's the black cylindrical part.
One end has an opening to fit a 4mm hex bit (this is the interface for all the other bits in the kit). The other end has a completely circular opening, slightly smaller than would take an H4. There's a stop in the middle, and it's magnetic.
I can't put the other driver heads from the kit in the round end, and it has no ability to apply any torque to anything that would connect, because it's round. What is it, and what is it for?
Best Answer
I believe the component you have is defective. I have what I believe is the same set of tools, and is the same brand name.
It is an extension for the main handle.
I am unable to separate the hexagonal portion from the black cylinder. The joined area of my tool appears to have epoxy or similar material bonding the two pieces together.
I would not have expected epoxy as the only bonding material, although I have yet to over-torque anything and have the bond break.
If you are able to insert the cylinder end on the hexagonal portion and engage some of the hex portion of the cylinder, you too can epoxy them together and have a working tool again.
EDIT: added two photos by request.