A form of urban dance usually (for reasons which will be readily apparent shortly) performed by women.
The essence of the twerk is the nature of matter to "bounce" when jerked back during a fall. This is not a new concept in dance - belly dancing is predicated in part in using a "hula" like motion to work with the natural bounce of hips and buttocks. Burlesque dancers would swing the body to bounce the breasts or make tassels on the ends of the nipples swing. But the twerk is consigned almost completely on the buttocks.
In this case the "twerking" practitioner typically stands back from a wall, presses her forearms and hands lightly against it, and raises on to her mid-foot. And then, in time to music, proceeds to "bounce" her buttocks by jerking the muscles of the lower back and gluteus to reverse the direction at the top and bottom of the bounce. In short, the buttocks simply move, and in the same way a yo-yo is kept moving by small jerking motions at the top and bottom of the yo-yo's path, the buttocks are made to drop and swing back upwards. Another popular position is to bend forward and rest ones forearms on one's own knees: but the importance is having some kind of ability to use the body's structure as a series of "springs" to bounce the buttocks.
Of course, the very best practitioners are quite callipygian and in truth the move is pretty dependent on some natural assets - because the pinnacle of twerking success looks almost effortless - the body moving imperceptibly, and the buttocks bouncing up and down rhythmically as if by themselves.
Very advanced practitioners can "twerk" in a circular or angular motion, either making the buttocks swing in small circles, sometimes one in one direction and another in another - or performing the infamous "booty clap" in which the bounce causes the buttocks to slap together.
Extremely advanced practitioners can twerk upside down in a handstand position, legs bent in a squatting position in mid-air.
The dance is most associated with people of color in the southern United states, especially in the Miami ("Booty Bass"), Atlanta ("crunk"), and New Orleans ("bounce") urban "hip-hop" subcultures. In fact "bounce" is most certainly designed for this type of dance, as it involves little more than an 808 beat box, choppy cut-up vocal sample and a distorted lead line added merely for flavor.
And as such, was recently the nasty focus point of some commentary on the state of people of color, cultural appropriation, and became a lightning rod of sorts - when Miley Cyrus, desperate to escape her Hannah Montana past and Disney Channel image - decided to appropriate the dance whilst using dancers of color as props at a VMA performance in 2013. And whereas the twerk is part of a longer, very deep rooted cultural tradition of dancing from the hips, she seemed to treat it as a very overt sexual display,
The key distinction here is that in the past, it was a case of the cart being after the horse - performers like Josephine Baker danced from the hips and scandalized Paris. Elvis Presley swung his hips and the Ed Sullivan show was BANNED from showing him anywhere near below the waist on camera. But in this case, it wasn't a case of "I have appropriated a culture and caused a scandal", it was a darker "I want to cause a scandal, so I am appropriating a culture". And if you follow that line of logic, and it can't be stated any other way except bluntly - "I want to be depraved and sexual, so I will act black."
This is where the real outrage came from. It was less so much that Miley showed up, stuck her tongue out and ground her buttocks (such as they were) against her co-performer, it was that she was performing, in essence, a minstrel show - exploiting a white cultural perception to give herself a desired image. This is not by accident, but by design. She stated explicitly, more than once, that she wanted a new "black" sound and a "black" image, which made it all the more depressing when she, for want of a better term, culturally appropriated and debased something and as a result slut shamed an entire culture. The most depressing part about it is, Miley Cyrus is a skinny white girl whose buttocks were tightly encased in latex, so whereas she could copy the movement, there's no way at all she could reproduce the effect. It was an empty performance of the worst sort. (This fact ended up getting some inverse racist hostility thrown her way, which led to the usual backlash.)
Using nonwhite people as props is not new either. Gwen Stefani surrounding herself with Japanese women as props (in her Harajuku) phase was bad enough - but then again white Tennesseans never owned Japanese women, which made the whole display far more sinister. Miley got her attention, and in an age without any shame and "no such thing as bad publicity" - got everything she wanted.
In the meantime, flamboyant "bounce" artist "Big Freedia", a drag Queen from New Orleans, ended up getting some publicity of her own - not that she necessarily needed or wanted to release a twerk-heavy dance video at exactly this time, but she did. And it got Esquire's attention. And the contrast couldn't be more intriguing - they fired Freedia back, glorious as she is - colorful (whereas Miley wore muted earth tones), energetic (there's quite the collection of bouncing women in the video), and - more importantly - actually able to twerk herself. Though obviously not as Callipygean as the women in the video, you clearly see Freedia join in and can in fact make what she has bounce.
Which really speaks to a certain cultural resilience. Eric Clapton made a fortune copying pentatonic guitar in guise of the blues, as a privileged white man with a lot of money ("woke up this morning... stocks up five percent?"). The aforementioned Elvis Presley was a marketer's dream - "a white man who sang and moved like a black man". Eminem had tremendous talent but being blonde certainly didn't hurt to propel Dr. Dre's beats into suburban bedrooms. But at the same time they're still playing the Delta blues and still singing about the injustice of living in a cold, hard society. There are strugglerappers in the "urban" parts of every city. And whereas Miley Cyrus will most certainly take her latest cultural bauble and throw it on the floor after wearing it once to the school dance, you can rest assured the twerk, in one form or another, will live on.