one thing all you guys are leaving out or missing is the twist of your torso , the reason you lose power when you kick standing straight up on post leg is that you minimized the whip , its all leg at that point , as sullivan80 pointed out that you lean back , but hunch head forward over the hips this allows your hips to turn over , but the most important thing that does is keep the whip flowing from your foot , up your leg , through your hips and torso twist completes the whipping motion of your kick ,,
watch ernesto again as there is lots of slow motion in that video ,, when he is in close and standing tall his kicks are mostly all leg , no torso twist , but see the whip when he gets his whole body involved ,,, the same principle is used with a left hook (punch ) the torso twists first then the punch follows, bringing all the power from the twist with it ,
hope this helps , its very hard to explain what im trying to say on a keyboard
to the ts ,, you already twist upper body pretty well , you just arent letting the torso go before the kick , work on that timing , also this is how you get power and whip with switch kicks or lead leg kicks , your arms and upper body must be part of it and timed correctly ,, same as a golf swing