That's French pop star Serge Gainsbourg on an old-timey variety show performing a tune called "Chez Les Ye-Ye" with an assist from one of the most enthusiastic backup dancers in recorded history. The moment that makes this video one of my favorites comes around the 0:55 mark, when Serge and his dancer friend both inexplicably pull out switchblades ...
They sure are good at being French.
... and then carry on as normal. I'd wear that video on a T-shirt if I could.
Anyway, sharp edges and live music will always be a weird combination, but that the knives in question here happen to be switchblades should come as no surprise at all. At that time (the song was recorded in 1963), the switchblade was the universal symbol of danger and violence in pop culture.
In the real world, their use was associated with gang violence. Magazines and newspapers throughout the 1950s were littered with unsettling tales of the violence and destruction gangs of switchblade-toting youths were inflicting on terrified citizens. The scare reached such a fever pitch that in 1958, the United States Congress enacted the Switchblade Knife Act, which effectively made the admittedly rad-looking weapon illegal to own.
Because of all this, I've had absolutely no reason to think about the legality of the switchblade. That all changed when I happened upon a website called uniqueblade.com. It doesn't matter how or why -- what matters is that I immediately noticed an interesting item listed for sale in the sidebar.