I have a similar product The Angler but find it more annoying than useful. The bobbin for my machine has a slide off top that you drop the bobbin into; you have to realign the angler each time you change a bobbin.
I see mckwilter and I posted at the same time; my angler does use the "key" to align it each time; It says it's a Pam Bono product, so it may not be available any longer.