to 柠檬椰子汁 (modified)

I respect ur posts and learn many things from them. Thanks.

I guess 水中捞月 is right. There is misunderstanding here. I modified this post.

I did not suggest them 转入地下 or 皇上面前一套，自己低下一套.

I suggest them to use that coach as a personal coach. I do not think "Doing personal coaching = “私下”打黑工".

I know in many sports, a player can have his/her personal coach in addition to his club coach. A person can choose to be or not to be a personal coach; that is his choice.

Hopefully my explanation makes it clear.

I just propose suggestions to help.

thanks again, 水中捞月, for your understanding.