Imo, there are so many factors in speed, it’s hard to pinpoint it. Separation of the chest and hips is probably #1 in my book. We know approximately 90% give or take of your speed comes directly from arm whip. Arm whip must haves imo are loose bent arm, upper arm stabilization and forearm deceleration (brush)
Separation helps facilitate 2 of those 3, if you have separation you won’t have to worry too much about upper arm stabilization or brush, as long as posture is good and they’re not over rotating the shoulders. I’ve included a top view of a 4D capture from a kid that can throw 65, note the chest (green line) more open to than the hips (red line) there’s a total of 18 degrees of separation in her chest and hips, stretching the fascia, pecs and sling producing explosive rubber band snap back velocity. Hopefully that makes sense.