I also read on a sprinting site that training (sprinting) on a stiff surface tends to make the tendons compliant, whereas training on a compliant surface tends to stiffen up the tendons... who knows if this is true.
You should film some depth drops from the same box height as in the video, focusing on not collapsing at all at the knee. Basically landing in a foot dorsiflexed position (weight over the whole foot) and not allowing collapse.
I know a lot of people recommend landing on the toes on the depth drops but I don't agree at all with that. That promotes the very problem you and I have right now. It's MUCH better to land on the whole foot while having them dorsiflexed and not allowing knee collapse/having the arms as if you're preparing to jump up immediately.
So try that - film some depth drops from the same height focusing on these things. If you're able to do them right then you can focus next on actually taking off from that position.
Yes you are, and that was predictable. I am as well. Basically horrible.
The question to ask is - why? Structure? Quad strength? Calf strength?
For me the obvious answer is the lack of quad strength. Hence my big difference between my low bar and high bar squat, my sucky "anything that has to do with the quads" thing (think front squats), I kind of struggle doing leg extensions with weights that other people that don't care about legs can do in my gym.
I have pretty good calf strength since I'm a one-leg jumper... you take the hips out in a depth jump so it's calf+quad.
You might say "yeah but I have a 2x low bar squat". My question is - what about high bar? What about a quad-driven squat? Because if you're good in putting out hip power in the low bar squat that will mask the lack of quad strength.
I don't know about you, but I will start doing high volumes of high bar squatting with light to moderate weights (think something like 4x10 with ~50% of my low bar 1RM).
Other than that you need to put yourself in situations that call for you maintaining that foot dorsiflexion, which is what consecutive hurdle jumps do.
1) The lack of dorsiflexion. You plant in a plantar flexed foot position. That leads us to number 2 2) The lack of dorsiflexion translates itself in the collapse of the knee. If you play it frame by frame you can see that you plant, go down further and then jump. It's basically a standing vertical jump with "momentum". You don't want that. You want only concentric movement after the plant.
This has been a culprit of mine for years. I have the tendency when I jump off two to go further down as I plant, but in reality you need to plant as low as you need from the get-go. You must not plant, bend down and then go up. You must plant and go up.
What that does is two things:
1) Keeps the GCT low (since you eliminate the eccentric phase); 2) Forces a rapid and powerful stretch control in the plant. Preventing the eccentric collapse recruits a ton of muscle and generates a ton of power in a CONCENTRIC manner.
If you're unable to do this from that box then maybe you should try a box that allows you to do this.
Viewing all this I have a feeling you'd be terrible if you'd try consecutive jumps to the backboard or consecutive hurdle jumps. If you have access to hurdles then that should be a priority for you. Why? Because if you try to plantar flex when you do consecutive hurdle jumps you won't be able to do them. They teach you to maintain foot dorsiflexion.
If you take all these things into account... to me it sounds like you're biomechanically missing in on a lot of potential VJ inches you have in you already. You just need to correct these things (me too, I have the same issues).