WELL, THAT SOUNDS TERRIFYING:

The skill -- a back handspring onto the vault then two-and-a-half twists in mid-air with a blind landing facing away from the apparatus -- is not for the faint-hearted.

It is a fiendishly daring and dangerous move but one she normally executes time and again with pinpont perfection.

On Tuesday, with her diminutive frame shouldering the crushing weight of expectation that comes with being one of the faces of the Games, the "twisties" struck.

Losing her sense of spatial awareness, with the real danger of doing herself serious harm, Biles bailed out of the Amanar in mid-air, turning just one-and-a-half times and stumbling on landing.

Fellow gymnasts have described the condition that interrupts normal communications between brain and body.

"You have absolutely no control over your body and what it does," US gymnast Aleah Finnegan explained on Twitter.

The loss of form is complex, difficult to treat, and can be compounded by pressure, a French gymnastics coach told AFP.

Any gymnast who falls victim to the twisties is "paralysed by the fear of losing" and serious injury, he said.

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