The tranquility of a warm autumn morning in Chiba Prefecture is broken by three shapes streaking through the sky above a rugby field.

They move so fast it is difficult for the eye to follow, flitting this way and that like angry dragonflies and filling the air with high-pitched shrieks.

A crowd stands watching below, protected by a row of soccer goal nets. In the middle sit three people in low-slung chairs, their eyes hidden by goggles and their face muscles twitching and reacting to pictures that only they can see.