Something "flipped on its head" has been figuratively turned upside down. It has been altered so profoundly that up is down, left is right, and a person's expectations have been completely overwhelmed.
Ex.: "Social media has turned the communications business on its head." Social media has revolutionized the communications business.
In The Heart Of
The "heart" of something is its center, its core, its most vital area. This can be figurative, or in relation to territory, it can be geographic in nature.
Ex.: "Protesters set fire to at least three police cars in different parts of the city, including one in the heart of the financial district." (Subject: G20 riots) This means, in the center/ the core of the financial district (of Toronto, in this case).
I Feel Like A Million Bucks
A buck is an informal American word for "a dollar." To say "I feel like" a million dollars means, the person feels worth a million dollars. The speaker is claiming to feel very good, in robust health and with a positive outlook.
Ex.: "I made the winning goal in our hockey game. I feel like a million bucks!"
On The Line
When your job is "on the line," it is in jeopardy. The job could be saved, but is in danger of being lost. That is, you might be fired if you do not do your job properly.
Ex.: "Matt's job was on the line due to his anti-social behavior. If he knew how to work in a team, his job would be secure."
Other things can be on the line, too: your marriage, your future, even your life. Anything "on the line" is in a crisis and could go in either direction (that is, up or down).
➤ Read more idioms and phrases, English grammar and vocabulary at Basic English Grammar And Vocabulary .
