Question: Which form of music is practiced in hardcore punk, reggae and ska and features guitar strumming techniques including the upstroke and bang? 

Context paragraphs:
Black Square (band): Black Square is a ska band from Honolulu, Hawaii.  Originally formed in 2002 as a hardcore punk trio, Black Square expanded into a six-piece and currently consists of Josh86 (vocals, guitar), Brian Kim (Drums), Sweet Taylor Baby Cakes (bass), Jared (trombone, backing vocals), T.R. (alto saxophone, backing vocals), and Babyface (tenor saxophone, backing vocals).  Since expanding to a six-piece set up Black Square has sought to blend ska, punk, rocksteady, reggae, and hip-hop influences to create a distinct ska sound that maintains the intensity and style of punk rock.  The band's name is a reference to Kazimir Malevich's 1915 oil painting of a black square on a plain piece of canvas.

Ska punk: Ska punk (also spelled ska-punk) is a fusion genre that combines ska music and punk rock music.  Ska-core (sometimes spelled skacore) is a subgenre of ska punk that blends ska with hardcore punk.  Early ska punk combined both 2 Tone and ska with hardcore punk.  Ska punk often features wind instruments and especially horns such as saxophones, trombones and trumpets, making the genre distinct from other forms of punk rock.  It is similar to traditional Jamaican ska, but faster and heavier.

Polyphonic strumming: Polyphonic strumming is a Nuevo Flamenco and classical guitar technique allowing for two lines of music to be concurrently performed.  While all varieties of strumming techniques allow for chord conveyance, only polyphonic strumming enables a simultaneous melody line carried out on the first, second, or both strings.  The sounds produced are markedly polyphonic, giving the impression of multiple instrumentation.

Reggae: Reggae ( ) is a music genre that originated in Jamaica in the late 1960s.  The term also denotes the modern popular music of Jamaica and its diaspora.  A 1968 single by Toots and the Maytals, "Do the Reggay" was the first popular song to use the word "reggae," effectively naming the genre and introducing it to a global audience.  While sometimes used in a broad sense to refer to most types of popular Jamaican dance music, the term "reggae" more properly denotes a particular music style that was strongly influenced by traditional mento as well as American jazz and rhythm and blues, especially the New Orleans R&B practiced by Fats Domino and Allen Toussaint, and evolved out of the earlier genres ska and rocksteady.  Reggae usually relates news, social gossip, and political comment.  Reggae spread into a commercialized jazz field, being known first as ‘Rudie Blues’, then ‘Ska’, later ‘Blue Beat’, and ‘Rock Steady’.  It is instantly recognizable from the counterpoint between the bass and drum downbeat, and the offbeat rhythm section.  The immediate origins of reggae were in ska and rock steady; from the latter, reggae took over the use of the bass as a percussion instrument.

Skank (dance): Skanking is a form of dancing practiced in the ska, ska punk, hardcore punk, reggae, jump-up (a drum and bass subgenre) and other music scenes.

Chaos UK: Chaos UK are an English punk rock band formed in 1979 in Portishead, near Bristol.  They emerged as part of the anarcho-punk scene, developing a fast and aggressive hardcore punk style.  The original line up of Simon Greenham on vocals, Andy on guitar, Chaos (aka "Lice") on bass and Potts on drums recorded two EP's and a full LP for Riot City Records.  In the process they along with fellow Bristolians Disorder and Stoke's Discharge revolutionised the hardcore punk scene.  In particular the Japanese 80's hardcore punk bands were heavily influenced by Chaos UK and Disorder's brutal take on punk.  Chaos UK's debut LP was notable in the fact that the band's label claimed it was the "fastest, noisiest LP in the cosmos" in the short lived "Punk Lives" magazine; vocal duties on this recording were also handled by bassist Chaos.

Ska stroke: The ska stroke or ska upstroke, skank or bang, is a guitar strumming technique that is used mostly in the performance of ska, rocksteady, and reggae music.  "Reggae is most easily recognized by...the skank."  Ska strokes serve as a rhythmic base to a song, and may be doubled by the drums.  This style of playing has a dance associated with it, the skank.  In reggae, the guitar usually plays a short, percussive, "scratchy chop sound [chord]," on beats 2 and 4 (1 2 3 4), often supported by staccato piano (late 1960s to the early 1980s) or synthesizer.

List of reggae festivals: This is a list of notable reggae festivals by country.  This list may have some overlap with list of jam band music festivals.  Reggae festivals may include classic reggae and related or derivative genres such as ska, dancehall, dub, hip hop, ragga, reggae fusion, and drum and bass.  Reggae originated in Jamaica in the late 1960s, influenced by Rastafarian culture, Jamaican dance music, traditional mento and calypso music, as well as American jazz and rhythm and blues, and evolved out of the earlier genres ska and rocksteady.  By the 1970s, large festivals in Jamaica were being held featuring notable reggae bands, and the Wonder Dream Concert in 1975 in Jamaica was one of the first internationally noted festivals to focus on reggae.  In 1979 Reggae Geel became the first reggae festival in Europe, and these concerts soon spread to other locales, becoming popular in regions such as Northern California.  With the introduction of the electronic reggae genre ragga in the 1980s, reggae began to be featured at electronic music festivals as well.

Canadian hardcore punk: Canadian hardcore punk originated in the early 1980s.  It was harder, faster, and heavier than the Canadian punk rock that preceded it.  Hardcore punk (usually referred to simply as hardcore) is a punk rock music genre and subculture that originated in the late 1970s.  The origin of the term "hardcore punk" is uncertain.  The Vancouver-based band D.O.A. may have helped to popularize the term with the title of their 1981 album, "Hardcore '81".  Hardcore historian Steven Blush said that the term "hardcore" is also a reference to the sense of being "fed up" with the existing punk and new wave music.  Blush also states that the term refers to "an extreme: the absolute most Punk."  One definition of the genre is "a form of exceptionally harsh punk rock."

Rasgueado: Rasgueado (also called Rageo (spelled so or Rajeo), Rasgueo or Rasgeo in Andalusian dialect and flamenco jargon, or even occasionally Rasqueado) is a guitar finger strumming technique commonly associated with flamenco guitar music.  It is also used in classical and other fingerstyle guitar picking techniques.  The rasgueado is executed using the fingers of the strumming hand in rhythmically precise, and often rapid, strumming patterns.  The important characteristic of this strumming style is the fingernail (outer) side of the finger tips (as opposed to their fleshy inner side) is also used, and in such case, in reverse of the way it is done when the fleshy side of the finger tips is used, namely downward (index, middle, ring and little finger) and upward (thumb).

Supporting paragraphs: Ska stroke Skank (dance)
Answer: Skanking