I know, it's fascinating, right? He generates so much power with really minimal movement.
I'm not an expert at putting forms but I'm pretty sure that it is a weight shift putt. He's really good at catching a tiny amount of momentum that he generates at his feet and leveraging it through the whole length of his body. He will do a little hip hinge on straddle putts to generate that momentum. He's got a video out about how he thinks the stance is the key to his putt.
As far as the downswing goes, he's probably loading up his posterior oblique sling (you can reference the numerous threads on this subject as it relates to backhand power in the forums) to unload along his aim line.
He seems to keep movement along the entire kinetic chain really minimal -- it's really just "tendon bounce" or whatever it is we call it when fascia rebound) along that sling -- until the transfer to his hand, which is under a lot of torque at the release. His release happens pretty low considering that he doesn't really drop the disc below much below waist height on stagger putts. The disc springs out and his finishing position is his hand in a karate chop position, with his fingers in line with his forearm straight at his aim point when he's really locked in.
The apex of his putt is pretty close to him rather than the basket (compare this to someone like Cale Leiviska who seems to peak out about 5' in front of the basket before dropping). It takes really bouncy wrist tendons to generate enough spin to keep it flat on that kind of trajectory. If you don't have the wrist strength to handle it, you will tend to have a nose down putt (side note -- there are a lot of really great nose-down putters, it's just not Calvin style).
His 2020 ITB has some comments about his putting grip.
I've tinkered with this putt a bit and the thing that has helped me develop the necessary wrist strength was to practice juggling discs -- throwing up/down without moving my feet, keeping the disc flat in the air. Palm down, low speed wrist curls will help with strength too.
Once you feel solid that you can get a good flat release, you can start adding the weight shift to propel it towards the basket.
