what are those "screams" that Serj makes especially on SOAD's first album?

Many songs on SOAD's first album have Serj's closed, muffled screams. Darts is full of this and I would like to know what it is, what it means and if there are other bands that use this technique. It's very strange that the music is happening, normal, and out of nowhere Serj says these things. It's weird, but I really like it.