The assistant referee held up his flag to show the referee that the ball was still in play.
Examples (Wiktionary)
Mandarin
从场上主裁判阿什比的角度来看，福勒是被西曼扑倒的，所以他对西曼处以极刑，红牌罚出场外，并罚点球。
從場上主裁判阿什比的角度來看，福勒是被西曼撲倒的，所以他對西曼處以極刑，紅牌罰出場外，並罰點球。
cóng chǎng shàng zhǔ cái pàn ā shí bǐ de jiǎo dù lái kàn ， fú lè shì bèi xī màn pū dǎo de ， suǒ yǐ tā duì xī màn chǔ yǐ jí xíng ， hóng pái fá chū chǎng wài ， bìng fá diǎn qiú ．
Seen from the angle of the referee, Ashby, Fowler fell because of Seaman's save, so Ashby meted out the most severe punishment to Seaman: he gave him a red card and ordered a penalty kick.