expression · to use a current topic as a starting point to express or elaborate on one's own ideas

他在演讲中借题发挥，谈论起自己对未来教育的看法。

expression · to use an incident or topic as a pretext to make a fuss or vent one's feelings

他其实是因为别的事不顺心，才借题发挥对同事大发脾气。