ㄉㄚˇㄅㄧㄢㄍㄨˇdǎbiāngǔ

打边鼓
  1. 旁邊比喻幫腔助長聲勢敲邊鼓」、打獵鼓」。

to echo what sb said, to back sb up from the sidelines (in an argument)​