Where did this come from??

I have gotten the UFO at least once, but there's no rhyme or reason as to how it works and it's not like it was rolled out with any fanfare or instructions, it just appeared. I understand it's performance-based depending on how you did inn the level you just played, but there doesn't seem to be any consistency to it and no idea how well u played a level gives me better progress or what...

Just another thing King has introduced to roll my eyes at.