We used to do this for fun in swim practice on our sides like that, and with fins on it's very easy to do what this person is doing at that speed. We always started races like this, the first 10-15 yards of the race were done out of your dive, under water doing fly kick. This is honestly just proper technique for fly, if you aren't activating your whole core, you're exerting more energy than you need. I always had a single kick butterfly stroke, not the 1.5 that most people do and you are supposed to come almost out of the water over your waves every time. This is why my butterfly was always faster than my freestyle, 46.4 second 100 fly vs. 48.17 freestyle. Butterfly is a faster stroke, it is just that no one can really hold pace with such a demanding stroke.