For those of you who don't know what the knife song is...
Many people who see this think it is highly hard and highly dangerous. It's actually pretty easy, just suspenseful to watch. Even more so if the knife you are using looks really deadly. The suspense may cause many people to refuse to do it outright, due to the perceived pain caused. While many people are so suspenseful that they actually do hit their hands, and that pain, or the blood, may prevent any future attempts.
But take the suspense away, and actually doing the knife song is actually pretty easy, and not all that dangerous, and scarcely painful, even if one does hit their hands.
This song has become popular, or in internet jargon “gone viral.” And due to that, many more people are trying it out and uploading videos of their attempts and successes for hopes are stealing their share of the spotlight. This further shows that this feat is quite possible to accomplish with minimal injury involved. The danger present isn't very large, the most obvious danger is getting a cut on your finger, the most common cuts are small flesh wounds. Unlike what the song said, if you hit your fingers, there is very little chance that they'll “come off.” So no worries there.
It's as much of a fad now as the cinnamon challenge was before, and about as dangerous (or not) too. This fad won't last long, and the only lasting scars you may receive is the that your friends and family will now think you are a psychotic masochist. Congrats!