tram-it, damn it

Does anyone know how a Widget called Tram-It works? (I think I've got
the name right.) Tram-It is supposed to help align the head of Bport
type mills. The thing is basically a 4"-6" disk about 1/2" thick with
a shank to mount it in the machine spindle.

You fire up Tram-It, wind the mill to 500 or so RPM and bring the
device about .050" from the table. The thing lights up around its
periphery (via multi-color LED's I presume), one color in the highest
zone and a second color elsewhere. You adjust the head until Tram-It
glows just a single color all around its edge.

A couple hundred $ is a little pricey when an indicator and bent rod
work fine, but it's a bit of a head scratcher how the thing works.

Flying .050" above the table, it wouldn't take me long to crash the
disk. Also I think I'd prefer not to adjust the head when the machine
is vibrating.

But how does it work?

Thanks in advance.

--Jack