This drill actual helped me quite bit. You snap the disc out of your hand, as high as you can, with the least amount of wobble as possible. It helps me get the muscle memory for on-axis wrist/finger action. It also trains you a bit to avoid wrist roll. The drill uses gravity to mimic the disc lagging behind the loosened wrist/fingers and helps to time the moment you activate them for the snap at the release of the disc.
https://www.youtube.com/watch?v=qiWvum5p1rg
Something to note.....In the drill the forearm is supinated a little further than in an actual sidearm throw where the disc is more parallel to the ground. Some tweaking is needed to get it to translate to an actual throw where forearm position is concerned. Its more about getting the wrist/finger snap action down for a more on-axis release