Like in this video, if you get your hips to lead in your swing while your hands are back you
WILL be a more powerfull hitter.
Straightleg
I would agree in general, hips first, hands last as to a sequence just as I would for throwing, as it is a way to generate a sense of timing and rhythm and momentum. This is the negative move or towards the catcher and the positive move towards the pitcher.
You do not have to slam the heel down to rotate the hips. Crystl demonstrates and shows her front foot heel being off the ground slightly when doing the matrix drill as it helps work her calf and hamstring and glutes. You will see her on the ball of her foot or what we term toe to toe and feeling the pressure between the knees as if fielding a ground ball.
Look up ground reaction forces on Google.
Efficiency however begins with how you load/coil and when the weight is more inside the back leg verses being over the leg in my opinion. Being able to simply lift the lead foot indicates the weight has been transferred to the back hip (glutes) in order to be able to stride. Keeping the movements simple especially with the younger hitters seems to work well as the develop their style.
If you stand with your shoulder against a wall and your feet directly under you, you can not lift a foot because you can not lean to shift your weight.
The studies that have been done concerning the neuro muscular skills of the females have indicated the boys are a little more advanced.
Combine that with the process of puberty and the female is going through a lot of differences in the changes to their body.
The Holy Grail to the female throwing and hitting better comes from landing on the ball of the foot with a flexed knee to throw. For hitting it is landing on the inside edge of the foot or what is termed eversion on a flexed front knee.
The glutes do not fire the knees in the female and it does in the male.
Think of the male knee working like more of a hinge and the female knee works more like a ball joint. The females center of gravity is also lower so the old baseball cues of stand taller in the box when you hit takes the efficiency out of the legs.
In my opinion this is why you see many hitters who appear to be upper body dominant because the swing does not start from the ground up.
Lovie Jung steps flat footed however can make her hips move efficiently. I have not seen many young ones that can master that move however.
How you get to toe touch and what happens after toe touch is where power is generated either efficiently or not as efficiently.