"
Along with that he said to keep my chest facing up as I made contact with the ball."
The chest faces more to the sky for a brief time. By impact, videos of high level servers show the chest is no longer facing up. Of course, describing this motion in words is always weak compared to high speed videos.
Google:
tennis serves impact pictures
Look for chest up at impact in these.
Look at the time when the chest faces up and how it faces at impact in high speed videos.
Google:
Tennis serve impact slow motion Youtube
The above examples show you two quick methods to check information that you are hearing.