He probably saw it being done. That's a really old dance from African countries, but it wasn't called twerking there. Like Mike's "moonwalk" wasn't originally called that. There's a different dance named the moonwalk in the 1960s. There's even a song about it by Joe Simon. The actual "twerk" dance can be seen in old 2 Live Crew videos from the 1980s and a lot of 1990s rap music videos. Janet Jackson is sort of twerking in her You Want This video, which was based on a 1960s movie about go-go dancers called Faster Pussycat Kill! Kill!.