I have seen them used for Crappie fishing.
They are more of a tip down, or work on the same principle.
You unreel the amount of line you want and then balance the "crossbar".
You can see when fish take the bait swimming up, or down, because the crossbar moves from it's original position.
They work o.k. but can be a Pain in the butt when it is windy.
Hope this helps,
Wiener