flash mobbing
noun/ˈflæʃmɒbɪŋ/
/ˈflæʃmɑːbɪŋ/
[uncountable]- an activity in which a large group of people arrange (by mobile phone or email) to gather together in a public place at exactly the same time, spend a short time doing something there and then quickly all leave at the same time
快闪族（通过手机或电邮相约同一时间在公共场所速聚速散的一大群人）