If a crowd of people do a Mexican wave, each person in the crowd stands up and puts their arms in the air after the person to one side of them, creating a continuous wave-like motion through the crowd.
[British]regional note: in AM, use wave
Mexican wave in British English
noun
the rippling effect produced when the spectators in successive sections of a sports stadium stand up while raising their arms and then sit down
Word origin
C20: so called because it was first widely seen at the World Cup in Mexico in 1986