So for this hitter, to be able to 'stay square' she looks like she actually stops or delays her momentum. perhaps from pushing back a bit on the front leg. Then once the delay created the correction of the timing, continued thru to a normal swing. maybe?
the front leg will adjust accordingly if the middle is loaded(no leaks)and anchored. Notice the young lady doesn’t sink as far into her front leg as the original pro hitter. he’s adjusting to a lower off speed pitch. the young lady is adjusting to something more ‘up’. But they both delayed launch through the middle of their bodies and met the ball a bit more ‘out front’. Essentially anchoring the lower half and letting the upper go ‘get’ or ‘meet’ the ball.