Like Zita above, I too started off with a Palmball, and to this day is still the only changeup I’m comfortable throwing regularly. To make it work for me, I place the ball directly in the palm of my hand and grip with all four fingers. The thumb is barely used at all. I also put most of my pressure on the index finger so I’m pressing the ball against my palm with the finger.
Now it may work differently or have a different feel for you. But for me, I find what helps is to release it similarly to how I release my knuckler. I let it roll off my index finger on release just for a moment, and then I push the ball off, taking all fingers off the ball. The end result should be a pitch that looks like a 4-seamer, and even has some rotation similar to the heater. But as it gets to the plate, it should just drop right through the zone.
If it helps, I included a picture of how I grip the thing…