When the harpist starts playing a song he tunes the strings according to the key of the song. Just imagine all the strings correspond to the white keys of a piano. Changing the hooks is the same as when you play the black keys on the piano.
The harp is a celtic harp, they have hooks. Big harps have pedals for the half-tones. What he does is by no means crazy or 'whizardy', my daughter used to play like this when she was 10 or 12. Like everything else, it's just love for what you are doing and practice.
Love and peace
Iðunn
Like Iðunn said, he's flipping little hooks back and forth, changing the string a half-tone down (or back up again). This is necessary to change from one key to another. The flipping of hooks is generally done between songs, to change to the key the specific song is in, but some music (like the theme of the Phantom of the Opera) has a lot of extra sharps and flats in the music, and those have to be played after flipping the hooks accordingly during play. And something like that, with lots of sharps and flats, is not necessarily the piece that a harpist might pick to play. It's not wizardry, no - but it's not the most common thing either.
The piece gains a lot of extra humour by the text, which is about chromatics and its evils - which is probably necessary to find it really funny, and crazy. One of the text bits, translated into English, is "I need more strings for this music - or maybe hooks, that might work too."