I used to believe that you should stand up straight and lean into all of your kicks. That way your weight gets behind the kick. However, recently I learned that you can get more power
and height into a back kick by hunching over when you kick. Leaning forward forces your hips back, which helps add thrust to the kick.
I have the StrikeMeter that I've mentioned in other threads - it gives you an arbitrary number that you can use to compare techniques of like height (i.e. compare headshots or compare body shots). Here's what I found:
Head shots: my kicks were about 10-20% stronger if I leaned back. This is in addition to other benefits you get from leaning back, such as:
- Easier to kick higher
- Gives you a counter-weight you can use to help drop your foot down faster (stand up straight brings the foot down quicker)
- Gets your head out of range of the opponent.
Body Shots: my kicks were about 40-60% stronger if I stood up straight.
The only thing that really surprises me is that it depends on the height of your target whether you want to lean back or not. What I think is happening is that for the higher kick, you're actually bringing your center of mass back in line with the kick, where with a straight kick you're kicking over your center of mass. You still are with leaning back, but at least now you've got the counterweight action going on. For the body shot, it seems the direct weight transfer is better than the counterweight.