He's playing with Freudian slips. Of course, what he's thinking about all the time is the master-slave game, and so here it comes out, when he's talking about his brother's diving: "He's a master diver. You really have to slave away at it."

他在开Freudian滑倒的玩笑，当然，他一直想的是,主人-奴隶的游戏，当谈到他兄弟的潜水时，他提到了：,“他是个跳水能手，你真得埋头苦干拼命练习这个了“

耶鲁公开课 - 1945年后的美国小说课程节选

It's very easy to use. But something you really should master.

这个句型很容易就能学会，这也是你必须得掌握的一个句型。

When 课堂 - SpeakingMax英语口语达人