One for efastball....

[rdbass]

The best hitter’s rear leg starts turning the quickest as barrel starts to move. Lower vs upper working against each other like gears.

Harper losing in the race, has to play catch up. - Bonds, Yelich are the closest to swinging at heel plant. Quickest trigger

 

[efastball]

hmmm, I'm not sure what you want me to say but here goes. I try my best not to use any big words or medical terms or confusing lingo.

I would say for most of today's hitters (not Donaldson and Bonds sometimes, and Bonds above):

[stride/move forward]
[coil inward] (most kids don't coil inward at all and it's usually a fatal flaw as they advance)
1a) hitters either start pre-coiled, or partially coiled or even or open, then they coil the torso (hips and shoulders) inward 40-45 degrees as they go forward
1b) they hold this coil in for most of the stride time/length

[the uncoil begins when the front foot is maybe an inch or two from plant, just a fraction before toe touch, so that you are very quickly back to even] (most kids don't get this right and begin to uncoil as soon as they start moving forward)
2a) the extension of the rear leg (with the heel on the ground) gets to almost full extension
2b) the smaller the radius of the knee bend, the quicker/faster it can rotate
3) the dropping/stretching of the front leg into toe touch begins a slow phase of uncoil
4) which begins the uncoil of hips and shoulders, but only advance back to even at moment of toe touch

[toe touch here]

[swing forward phase begins]
5) the turning of the front knee (usually during heel drop, but some turn an instant before landing on a flat foot) begins the rapid phase of uncoil
5b) uncoil above begins the rear hip to rotate and go downward (some kids only rotate level here)
6) which begins the elbow drop (most kids do this before toe touch, in mid-air)
7) which makes the bat head go backward toward the catcher [it's optimal if, at toe touch, the rear forearm is mostly horizontal, the bat head is up, and there is no slack in the top hand grip] (many kids do not move the bat head at all when the elbow drops, losing early bat speed)
5-7 probably all happens at the same time, but if they don't they happen in this order, if anyone cares.
8) all of this gets you to hip slot (elbow hits the hip)

[front leg/knee finishes torso rotation]
9) then the front knee/leg straightens to finish the rotation
10) the torso finishes the last half of rotation, with the front bicep as close as possible (many youth stop their rotation here which forces the hands forward, and keeps the bat head from passing the hands)

For the few other hitters like Bonds #755, and above, plus sometimes Donaldson, probably always Gwynn, etc. I have not spent thousands of hours on them as I have on the others, but:
1) I think they begin the uncoil by consciously turning and rotating a very bent back knee (with a very large radius - very slow, but they seem to time the pitch with this)
2) they mostly power the torso rotation with the straightening of the front knee/leg

All the above use the Conservation of Angular Momentum (sorry, I broke my rule here, just think of a skater pulling their arms in and out to slow down or speed up) to rotate as quickly as possible to contact. This means they keep both biceps as close to the torso as long and as much as practical during the swing forward phase. [the length of the bat, plus the forearms, will enable you to reach any pitch up/down/in/out over the plate]

I am editing this post as I clarify. This is the first time I'm trying to describe this here in this much detail. [#3 and 5 have been modified]