From the description:

Even though these Belgian records sound very "now", they are actually 20 years old and were meant to be played at a much, much faster speed. At the time this was the devil's music for us, but we have learned to listen through the claps and distorted kicks and discovered that if you slow these really dark and heavy techno records down all the way to about 115 bpm, it suddenly makes them sound less frantic, ballsier and a lot sexier. Belgium at its best when pitched down.

This is apparently how Belgian new-beat started from techno records, although it was in the eighties, before the time of gabber — so this video is anachronistic and is more of an experiment in recreating the process.

Famously, Aphex Twin advised to do about the same with his ‘Richard D. James Album’ in reply to complaints that it's too short — saying to play it at 33⅓ rpm instead of 45 to get a full-length album. The same approach works great with some other idm or hard-edm records, making them mellower and more relaxed.