You start with your arm in the reachback position and leave it there.
You begin your hip rotation and that will pull your arm a little towards your chest as your pivot your hips.
Then, you begin your torso and shoulder rotation. That will pull your arm even closer. It's at this point, when your torso/shoulder is pulling your arm, that you want to accelerate your pull and time it so that the disc rips from your hand just when you square your shoulders up to your target. That's what makes it so hard.
If you over accelerate fast or rotate too slow, then you get an early release. if you over accelerate or rotate too fast, then you get a late release or grip lock.
If you keep your chin over your toes, then your shot should be hyzer or flat, if you stand upright with your chin over your toes, then you'll end up with flat to anhyzer (which could make your flat shot turn over, too).
Depending on if you're throwing elbow or shoulder lead dictates which you lead with, but with either throwing technique, you need your elbow to hit your target direction first, then your shoulders should begin to square up as the disc tracks down your arm and eventually rips out as your wrist pops open just as your shoulder square up.
Your speed and power come from your arm being the last thing you pull in your throw, not the first. If you start with the hips, then add your torso, then shoulders, you've already got a ton of speed and power built up and your arm should already be in motion. Now you're not bringing your pull through from a dead stop, you've already got 20% of your speed going without using your arms. Then, when you feel your arm moving, you need to actually pull as fast as you can. That's when you're maximizing your body potential by utilizing your bigger muscles to generate your power.
The problem is that when you do this after not doing it, you'll find that you rotate much faster and before and will release late a lot until you get the timing down. Your arm speed will probably feel super slow as well because you're not used to the speed you are generating.
Many, many players know this, feel this and just do it naturally. For those of us who haven't performed these type of timing actions before (throwing baseballs, footballs, batting, etc.) then it's more difficult because you need to teach yourself these fundamentals, practice them, and perfect them. Most people don't understand the throwing mechanics, let alone practice them.