The part of the threader that does the work is a flat piece of bent metal that slips through the eye of the needle. It is sturdier and easier to see than the typical fine wire that is in most threaders.
The flat piece of metal is big enough that it only works on a big needle with a long eye. It only works with one of the needles that I carry, but that's enough for me :-).
This threader fits the #5 Tulip Embroidery Needle. I use it with #12 and #16 perle cotton and floss. You might also be able to use it with thicker threads and this needle, but these are the threads I have threaded successfully.
Click here to see a video about larger needles, thicker threads, and needle threaders.