Regrettably, I read Facebook posts. Recently I saw a question posed about whether to punch with your back leg locked or bent. While I did not read all the comments, it appeared that many of the respondents said you could do it either way, and some laid out the corresponding conditions as to when. That was good to see, versus a strict adherence to one way.
I’ve addressed this in the past but it’s worth revisiting. A rudimentary rule I’ve taught is that you hit with the leg straight for penetration while you use the bent leg for speed. This does not preclude using one or the other either.
One of the reasons there are so many demonstrations of the use of both types, with a variety of stances and situations, is that there’s more than one way to do it.
One reason is that some people like it one way better, i.e. tailoring.
Mechanically, when the returning force (for every action, there’s an equal and opposite reaction), makes the trip back, the heel being up tends to “soak up” some of the power. The flex may give you the opportunity to control that force just a bit more.
The rigid rear leg really adds a brace that helps transmit the force to the target. One reason for learning proper breathing is that tensing the abdominals helps to make your muscles act like a weight belt and brace your core, since the lower spine is a weak link in the kinetic chain.
Sometimes, the heel-up helps when striking on a downline, it adds a bit of marriage of gravity by allowing you to drop just a bit more. I think, conversely, that striking on an upline, you’d want the leg straight.
The lines will also dictate whether you use a close versus wide kneel. Another thing to consider is whether size or muscle strength is an issue. I mean, can you handle that returning force? Are you strong enough? Hence the necessity of lower body training with stance and maneuver work. (And bag work.)
Just my two-cents.