Are neck cranks, the Twister, bicep and calf cutters illegal in bjj competition?
Generally they are, until blue or purple belt ranking, at which point all neck cranks, includint the Twister, knee bars, muscle splicers and compression locks, including bicep splicers and calf cutters, and chokes are literally fair game. This of course is just a commonality, but is pretty much standard worlwide. The reason, for this is because attacking these joints and muscles with submissions is considerably more dangerous than using armbars, which is why Kodokan Judo rules forbid the use of any jointlock that doesn’t attack the arm or shoulder, it’s for safety reasons.
