Why does 定 (dìng) disable the monsters' movement in the movie 捉妖记 (Monster Hunt)?
I know the magic "定身法"(the body-immobilization magic) from the very famous TV show 《西游记》, in which the protagonist "孙悟空" uses it to immobilize monsters. The magic spell he uses is a single character "定". It went viral since then. Kids would joke around about it. 

Nowadays, almost every native speaker knows this magic, given that 《西游记》 is such a popular TV show as well as its book.