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?