I don’t think this is what is happening. He goes through the string, he’s not bouncing off it. I think people just think that because the motion appears to make a circle. When these motions are fast, they are fast because the muscles being used can operate really quickly, not because the string is returning some kind of energy to the pick. I’m pretty certain of this.
What you’re looking for is a motion that goes fast immediately, even if it’s only for short bursts. But I would caution this needs to be more than just two or three pickstrokes. That’s not a good enough test. You should be able to do at least a bar or two of very fast downstrokes or upstrokes, at the target speed, or it’s probably not the right technique.
Here’s an older attempt at downstrokes using a form that is more similar to yours. It’s about 220bpm. When I got it, it was fast immediately, and felt really easy. But as you can see in the clip, sometimes you just lose the coordination and freeze up. That’s how these things go:
https://www.instagram.com/p/B9AQ849n8VK/
As a general note, it’s ok if you feel like it’s random and you can’t activate the technique every time on command. And it’s ok if you feel like every muscle in your upper body is hulking out at first. That’s the part that you will quiet down with training. But you have to be hitting the target speed right away. Ideally, way beyond that, since MOP is somewhere in the mid 200-teens and the actual upper limit for this type of thing when done with the most efficient technique is substantially higher than that.
Also, when in doubt, try upstrokes! They can sound really good, and be freakishly easy and fast. See this thread for some examples: