Proprioception is the key
The question had to do with why a juggler keeps looking up while juggling many objects (such as balls or clubs). The objects being juggled move up and down in the air, but the juggler maintains his visual focus upwards.
The answer, which would surprise most people, is: proprioception.
This phenomenon is explained in a previous blog post:
I have actually gone a step further - Juggling Blind (absolutely no looking). In the following video, I used a bucket that is opaque. This is a real feat that is achieved through practice and it is not an "illusion". Watch the video here: