Very very thin shavings! As in thinner than hundredths. Anything bigger and you will be disappointed with the results.
If you have never used one they take a little getting used to.
What I always do is but the piece to be cut up to the face of the blade then move the blade and advance the piece less than a hair forward. Preferrably an icelandic hair and not a mediterranean one.
I have one of those, but don't have the extension/attachment. Looks like the Grizz one might fit, though. (Mine's an older "original.")
I use mine fairly often for fitting frames, etc. It got a real workout on that pyramid jewelry box I posted in the 'Fair fair results' thread. That box has 24 mitered corners in it, if I remember correctly.
As you probably already know, fitting miters can be tricky - particularly when fitting four of them into a rectangle where the individual legs have to be exactly the same length. The trimmer makes that job a lot easier because you can trim just a few thousandths at a time to fine tune the lengths.
BTW, a cut of ten thousandths of an inch (0.254mm) is a huge bite to take with one of these. See-thru shavings are the best way to go.
If you want to trim a fine hair, don't worry about the Icelandic hair. Cut it as thin as my hair. Most of my hair is so thin that you can not even see it or feel it.
Make sure you pick it up my the handle also. I decided to move mine futher back on a shelf and one finger has never bent again.
They have increased in price since I bought mine...looks like time for the bay.