Forming ("chasing") internal threads?

Snap-On makes this internal thread chasing tool:
It comes with what seems to be a selection of tips in different t.p.i. but
not diameters.
Since there are no diameter specs, I presume that these tips don't span the
entire diameter of the hole, but lay against an "arc" of the diameter.
As best I can figure you place the tip (the thread edge) into the threads and
kind of scrape the threads. Unlike a tap or a chaser made for a specific
diameter threaded hole, this tool seems to rely on a bit of side-force to
affect the threads.
How do you use a tool like this?