Sinkers don’t spin forward, it’s the backspin and tilted spin axis that creates the movement. Curve balls have top spin because of finger placement but your fingers, for a lack of a better phrasing, will be behind the ball on a sinker. The goal when throwing one is to have it come off your middle finger so that you create the proper rotation, where four seams come off both and breaking balls come off pointer fingers because they’re supinated. A good video to watch to get a better understanding about pitch types and spin is this by Trevor Bauer:
Hope this helped.