Most people think the leg kick has to do with the players load. In truth it has to do with timing. In order to hit a pitcher stronger than you, the worst thing is to to get your foot down early. The moment your front foot hits the ground it is now in the way of your hips and shuts down your lower half. This causes players power to drop (due to lack of hip rotation), an off balance swing (hips are stronger than knee and forces you to fall off), and to chase pitches out for the zone (you can no longer reach the outside part of the plate).
What you should do to combat a power pitcher is to start your load early. A players load should begin at hand separation of the pitcher and at full load position to move forward at ball release. If the player has a high leg kick like Donaldson this will allows him to see the ball longer in hopes to identify the pitch and adjust to the off speed pitch. If it is off speed he simply lowers his leg slower allowing his kinetic chain never to be out of sink.
Having full power and being on time to the pitch.