My 2 cents...borrowed from someone else but I agree 100%
The rear leg snaps, not the hips.
You use your entire torso to load the rear leg. I give the cue of picturing your torso is a rigid block. Fold your torso diagonally around your rear leg - create a crease at the groin. Then add the scap to further load the rear leg. Your rear foot should feel like it's getting screwed into the ground. Your leg should feel like it is loaded like a spring (spiral like). If you feel it in the knee you are doing it wrong and should immediately stop. Your scap and thus your hands are now connected to your rear leg load.
You can load into the rear leg and you will just push out of it to swing. You can load your scap and just push out of that to swing. Your goal is to load the rear leg in such a fashion that when your launch the swing with your hands your rear leg will snap (unload). You never try to release the scap from the spine.
Some will say lower the lead shoulder and while that will give a certain look it will not automatically load the rear leg.
Working on making a couple of clips to illustrate the rear leg load - here is one
Edit to add another
Brett started in that position and then increased the scap pull back - you can clearly see the crease at the groin
A couple of comments. First, it has been my experience that if you spend a lot of time teaching GRF, you can mess a hitter up. For me, I like to see it happen naturally and then comment on what a hitter felt or what they tried to do elsewhere in the swing that helped get the results on the lower body/ground. Secondly, one area that I do have my hitters work a lot on is the concept that the back hip straightens the front knee. We do that with either those taller 4 step step ladders or now up in our balcony, we have some really high backed chairs. Finally, Charlie Lau used to comment about hitting with a platform. IMO, that platform is the result of resistance on the back side giving way to the other pressures and then, in a sense, holding that hitter in place with that rigid front leg/straightened knee. As my buddy Mudders always say, YMMV. Not an expert.
Most important components of GRF for hitting are the ones which are parallel to the ground, e.g. the frictional forces, which are related to the one which is normal to the ground via the coefficient
of friction. These are the forces which will resist against the parallel to the ground loads which the foot will exert due to the coiling of the hip and back. With little to no resistance of these
parallel to the ground forces and the foot will slip as soon as you try to coil.
As an example coefficient of friction for steel against soil is about an order of magnitude larger than steel on ice. I would expect shoe with cleats against soil would be an even