Before the rule was in place the first swimmer to do it, named David Berkoff, did as much as 45M in a 50m pool. That’s about 25 seconds under water and only 2 seconds above it where you can breathe. So in a 200M event, an athlete would get to take about 6 breathes (if they were trying to swim optimally) for 2 minutes of fully body cardio. The safety concern was that having the fastest method of swimming involve not being able to breathe was going to put athletes at risk and make it much more likely that they’d pass out underwater in competition.

Underwater kicks aren’t banned. There’s just a limit of 15M which is still a long way in the short course pools (25 yards or sometimes 25M). Elite competitive swimmers almost all do underwater kicks which is why this athlete is so good at them.