@HDMI1
"And he loses his left leg with that move since it's fully extended towards the point of impact."
If you look at the drawing well you can see the knees in a forward position relative to the heel and the upper thigh
It is only 30 meters high, for someone who blocks the arm of a wyvern it is not much, especially that the wyrvern will absorb the fall with his head
and I doubt that with a single wing blocked the wyvern falls like a stone
but it is true that the most logical thing would be to jump up at the last moment to eliminate the most speed, before the impact or to be at the last place that will touch the ground
Personally what bothers me is how he blocks an arm without a point of support
this position exists, but normally we use the ground as supports
this position does not allow the wing to be held against the body
moreover there is not even a joint between the legs and the arms of the MC
on page 14 we can see him having a point of support with his buttocks and it doesn't really prevent movement,
because for that it would be necessary to prevent the return to the starting position, with the left leg under the body for example.
and even if it did that is not enough to block the movement in the other direction,
for that it would be necessary to have the right leg along the body of the wyrvern
Basically it does not block what could be called the elbow, because it is higher and does not block the shoulder because it has no supports
so it does not block any joints, it just makes dead weight and it hinders movement range a little
it is enough to bring down the wyvern, but that does not prevent it from flying to the ground
If in the head of the author, if it is necessary that the wyrvern falls like a stone, kill her in the sky directly, there it will fall like a stone
(frankly when you have the strength to jump to 5 10 meters high and the force sufficient to counter muscles two to three times longer and bigger than yours, it should not be complicated to kill this beast)
If you just want to reduce movement and want to fall more slowly to survive (don't call it "armlock", when the arm is not blocked)