Scott I picked up on the water from Bruce Lees book as he says become the water my friend.
We say slow to load and soft to step on a flexed front knee and separate the hands at or prior to toe touch. I look for where the lead foot leg knee cap is pointing. As right handed hitter IF it is pointed towards the first base dugout I see the weight shift stopping or at least slowing down. If I see the knee cap pointing towards the second base mans playing position you can see and feel that springiness in the hips. I actually have them get to toe touch and gently slap the lead leg thigh and you will feel them move much like Jello on a plate. Then I have them stiffen the leg and they feel the movement stop. This may be the mass you are talking about as it slows down and in my opinion you are getting out over the leg and out of sequence.
Another way to get them to feel the core is we have them load and get to toe touch and I rotate them by putting my hands on both hips (you can feel the bony protrusions of the hip) tell them they may be unbalanced and to grab your shoulder if they feel like they are going to fall. Doing this from a static standpoint without feeling that flowing action of weight transfer may be why. Make sure the knee stays flexed and then have them make it rigid like they were at contact. Now have them put one hand on their stomach and you put two fingers just below the clavicle notch ( the V shaped area below the chin top of the shoulder area) then put two fingers at the or just below their belt line at the waist (basically the base of the spine) push forward at the waist line and hold them back at the clavicle notch. They have to work with you and firm up the front side and they will arch their back a little and the head moves back a little and ask them if their stomach area is tight now? It should be as the core is engaged. Now slowly put the hand that was at the belt line in the middle of the shoulder blade area and begin to tilt them forward just a little like their weight was over the front leg a little and their stomach ( core area) will get soft and they can feel it. At contact we want them to feel that core are engaged and unless they are lying to me they claim they can.
For me it starts with being balanced and at clinics we demonstrate this with the seasoned coach who knows it all and have fun doing it. We ask them to show us what balance looks like? We have a saying we try to make everything measurable and observable and then how can we test to see if you can feel the difference. I will push on their back and chest alternately and they will rock up on their toes or heels. Now in this sequence have them bend at the waist first and soften the knees a little and try it again and you will not be able to push them. To demonstrate it (balance ) only works one way have them soften the knees first and bend at the waist and you will be able to push them backwards so be careful! This is why I never use posture to explain balance....balance is balance and I have never found another way to do it. If you walked into a room and saw me you could not tell if I was balanced correctly until you pushed on my back or chest. However you may see what you think looks like you are in a balanced posture however you would not know it until you tested for it.
Most kids who play basketball sink to low and are slower because they have to raise up to get moving and as Kobata says too slow, not quick enough!
The bottom line is we want them thinking flowing to the ball like the water. Water is allusive and hard to stop or contain. Try to grab a handful of water, slap it, hit it with your fist and as Bruce said become the water my friend for when you put it into a teapot it becomes the teapot!
Slow to load, soft to step on a flexed front knee, separating the hands rearward slightly landing on the inside edge of the lead foot (eversion) allowing the weight to shift and changing direction at or during heel plant and you can find many do it a little differently and think driving from the big toe on the back foot as it is a basic martial arts move from the ground up. Put a concrete block on the outside edge of the back foot and they catch on quickly and they will not squish the bug!
