To be clear you are making the straight leg statement for Poomase only, correct? There is no way the standing leg can or should Always be straight during sparring and such.
Yes, but I wouldn't call it "for Poomsae" but the 'basic performance' of a Sidekick.
Everything we learn in "Kibon Dongjak", in the 'basic techniques', can and must be adapted to a particular situation. Just as we wouldn't spar in traditional stances or punch from the hip in sparring, the supporting leg doesn't need to be straight in sparring.
From a TKD perspective... what is the reason for the straight supporting leg?
If you ask some they may point out that "That's what the Poomsae key points of World Taekwondo say", if you ask others they may say "It looks better".
While I agree with both I would add: The reason for the straight supporting leg is to make it look different from Karate.
There are many ways of executing techniques and a lot of times there's no right or wrong. There's only right or wrong when a certain ruleset or technical standards get applied. And while we work hard on the perfection of certain techniques, we need standards to go by. And especially when it comes to Poomsae competition you need to standardize techniques. Otherwise it would be even harder to judge than it already is.
Plus it's part of the never-ending process of learning. When someone learns the sidekick there are other things to focus on first. Balance, flexibility, correct chambering, kicking in a straight line, hitting with the right part of the foot, foot positioning, turn of supporting leg/foot, turning on the ball of the foot, controlling the arms, control of force and speed. There are a lot of aspects to consider and the straight supporting leg is one of the less important ones I would say. When I saw the video of the sidekick in the video in the original post, there were other things that I noticed first. I didn't really pay attention on the supporting leg first.
