[chéng xū dǎo xì]   

乘    虚    蹈    隙    犹言切中窾窍。比喻叩击中节。

犹言切中窾窍。比喻叩击中节。

Take advantage of sb.'s undefended state to attack

乘龙配凤    乘鸾跨凤    乘风转舵    乘间投隙    乘机打劫