The beginning of stroke carry the most power, he has some nice stroke at the beginning, but lack of power. The power is transfer using the wrist movement (snap action) at the highest point. Then comes the leg.
So here is what I suggest:
1.
Train the shoulder and triceps muscle. Start by shoulder press. You be amazed how much it helps just after 3-5 sessions.
2. Flex the wrist. Snap action is not thorough, as it doesn't snap all the way. Technique is very critical here to avoid injury.
3. Instead of using the right leg to push the body upward to catch the bird; try not to jump, but stand firm with more weight on the left leg, like 60-70% of body weight. Consciously remind yourself before hitting the shuttle. Use a bit of body rotation if necessary.
Small jump is good to add momentum using body weight.
But it would be a waste of energy if it cannot be fully utilise. Imagine this: try stand with one foot (another in the air), and use that foot to
jump and rotate the body at the same time. It is just too difficult! I know because I have the same problem.
I find the first two steps easier for me, and still playing to improve my stroke in that area. I do jump,but just to gain better angle of attack than power.