How does this Steerer Tube Plug work?
 

Turning the large black thing attached to the metal cylinder causes the cylinder to expand, but I can't do that while the cylinder is in the steerer tube. I'm lost.

Thanks,
B.

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It looks like there is a hex recess in the bottom of the plug. Does turning that expand the plug? Can you get to that when the plug is in the steerer tube?

You insert the part on the right side of the picture and tighten it the (probably 6mm) screw firmly. Don't go crazy you just don't want it to slip when adjusting the head set)
Now install the top cap and screw and adjust the head set.
Note: Sometimes this style is a bit slippy when installing the plug part. so you want to adjust it so it can just be pushed into place, other wise it will just spin as you are trying to tighten the expander

Make sure it's loosely snug when you insert it into the steerer tube.

For some of these compressor plugs, I've found that it helps to grease the tapered plunger part and the inner part of the expander (so that it expands more easily; and contracts more easily when you've to take it out).