There was a short article about this in Flyer.
To avoid flying into each other apparently any aircraft that gets another in sight has to call "sighted" over the radio. If you lost sight, you have to call "unsighted". As soon as both aircraft are unsighted, the aircraft have to break away somehow and the race is voided. Or something like that.
Yeah right...
As far as I know there have been three incidents now at the Red Bull Air Race where a pilot got well within less than an inch of his life. (I'm only following things from a distance so there may well be more.) That was when racing solo without having to keep track of anybody else and without having to use the radio. And that race season is now suspended to make improvements "to the already high level of safety".
Press :: Red Bull Air Race