"Fever mode" refers to a state of heightened energy and enthusiasm. It is often used to describe a person who is highly motivated and excited about something, like a project or a task. Full definition
This mode also points out where you should start to set off a chain if you get into fever mode, which helps out quite a bit for new players. (hardcoregaming101.net)
I am in full spring fever mode, and I am no stranger to decorating pots. (lifelovelarson.com)
When this reaches 1000, you can activate a special fever mode where all destroyed enemies will give an exponentially larger point value. (hardcoregaming101.net)