airgrinder
What your proshop operator is referring to is to keep the edges of
your thumb inserts more angular. I.e. you bevel it less when
bevelling the edges, if at all.
It's supposed to make squeezing the ball painful at the base of
one's thumb because the sharp edge will cut into one's flesh if
he/she squeezes the ball too tight. This will in turn build a
proper release if one is formerly guilty of an improper grip, or
will make an otherwise average grip release the thumb earlier
because it is simply too painful to hang on to.
I know a few people who use this kind of equipment, some more
extreme than others. And I cannot throw their ball at all because I
don't like the sharp edge biting into my hand during the downswing.
Perhaps I have a tight grip, but I function fine with it. 
You can always try a ball from someone who uses this before
spending your ca$h.
