Hello Jocke,
That tool takes some practice to master, but can ultimately give you superior results and better fitting screwdriver blades.
The ideal blade shape takes some time and a lot of adjustment and trial and error to get to, but should look something like the blade in the image below.
With a lot of use you'll find that you'll create flat spots on the grinding stones, so you'll want to rotate them frequently, especially if you're sharpening large blades from scratch. Also, an ideal blade shape would have a different curvature for a different diameter blade and that's not really possible, so you'll find that very small blades don't have much curvature.
Enjoy your new tool!
_john