My cat winds her head around while playing- what does it mean?

She sort of flicks her head back and forth before striking at the toy. I've had cats for my entire life- 25 years- and she's the first one whose ever done this. I'm sure it's normal behaviour but I can't find the right words to use to search for it, so I'm asking here. It's sort of like how a person would wind their hips while dancing- back and forth in a really fluid motion. It's not to watch the toy, because her movements take the toys out of her line of vision.

I'm just curious to know if there's something beyond "I'm gonna bite that!" behind the motion.