The pattern of movements and speech you’ve described are part of a subculture of blacks who use more colorful slang than their (typically older and better-educated) black counterparts. It’s used for self-expression and out of habit, depending on the person. You’ll see a lot of this on talk shows because talk show audiences are not typically populated by well-educated people, but by high school students skipping school for the day or other young, sometimes lower/working-class people. The people on stage are generally from the same socio-economic class.
‘Know what I’m sayin’ is definitely in vogue among the youth/hip-hop set in general – turn on the Korean or Chinese Top 20 videos and you’ll hear it coming out of the mouths of Asians, turn on MTV and God only knows who you’ll hear it from.