Final answer:
The dance craze launched in 2017 when Russell Horning performed it live during a Katy Perry appearance on 'Saturday Night Live' is known as the floss dance.
Step-by-step explanation:
The dance craze that was launched in 2017 when Russell Horning–aka 'Backpack Kid'–performed it live during a Katy Perry appearance on 'Saturday Night Live' is known as the 'floss dance' or 'the floss'.
The floss dance involves swinging your hips in one direction while swinging your arms in the opposite direction, mimicking the motion of flossing.
It gained popularity after Russell's performance and became a viral dance trend.Unlike historic dance crazes like the Dancing Mania of the Middle Ages, which spread due to psychological and social factors, the floss dance gained popularity through social media and online sharing platforms, which allowed it to reach a wide audience quickly.