"The Struggle is the Sifu" My weakness needs work

So far I've have been involved in Kettlebell Sport for almost 4 years now. It started out as a hobby and something different to do when I was just getting into Kettlebells. I am interested in a lot of different things and I like to keep myself educated in all aspects of fitness. I first got into kettlebells after I graduated from Lock Haven University as a way to get stronger and not bigger because I was boxing. From there I shifted to doing kettlebells full time because I just found the workouts with this tool to be so much better than other tools in the gym and it was different.
Onto kettlebell sport....the more I looked and found out about kettlebells I ran into girevoy sport. A competition in weight lifting for time and reps compared to boxing where you get punched in the face. Easy choice to make for me. So when I first started I didn't know what I getting into but I was interested. So I did the AKC/WKC coaches certification which was a start but only that. I started posting videos and was contacted by Chris Duffey. He took me under his wing and I've been there ever since.
So being that I have been doing kettlebell sport for a while I think I'm starting to get the hang of it. It's taken some time but I'm starting to see the progress and the work I've been putting in has gotten me to the point where I'm comfortable.
The struggle is the SIFU especially when you are in a timed event and you have to produce the best result. Biathlon 10 mins of Kettlebell Jerk and Kettlebell Snatch- tough as hell! You get better the more time you spend under the bell. My struggle is the rack position in Jerk and since I've injured my hip last year the rack position has gotten harder. Keeping the leg straight is my biggest weakness. The legs do a lot in the jerk.

 

Being able to keep them relaxed in rack allows them to recover and prepare for the next rep. As I go longer into my set my knees start to bend and my quads don't relax and hamstrings are firing and the pop from my legs start to get weaker and weaker and my upper body takes most of the work to get the bells up.

 
As seen here knees are slightly bent and that can cause the quads to lose its juice and the body not to rest. Moving at a faster rpm and lifting heavier bells causes this to happen earlier into the set.
So the goal for me will be to build up my leg endurance as well as flexibility and mobility in my hamstrings, hips and quads and can't for the spine and back.  So what I am focused on whenever I train Jerk is the correct position for rack. Which means more training in that position as well as a focus of flexibility and mobility on days I don't train Jerk. Here is what the correct position should look like.

Legs straight and back relaxed so proper mechanics of the Jerk can be applied for high repetition.

Here is video of my last training where I work on just these things. Interval training with increasing weight. 3 mins on/ 2 mins of rest x 3.