From the Nathan Deakes Website
Race Walking is classified as a "technical sport". This means it is judged in order to enforce rules. Judges are positioned around a course to ensure that competitors comply with the rules.
Electronic aids are not used in judging competitions, because the rules stipulate any breach of the regulations must be visible to the naked eye.
There are two rules:
Loss of contact: the athlete must have one foot on the ground at all times.
Bent knee: the moment the advancing foot touches the ground until the leg has reached a vertical position, that leg must be straight (ie. not bent at the knee).
When a judge sees an athlete in danger of breaking a rules, he or she may caution the athlete. This is done by displaying a yellow paddle with the offence identified by a symbol on each side - (>) for knees or (~) for contact.
When a judge considers an athlete to be breaking a rule, the judge issues the athlete with a warning.
This is non-verbal, but at championships, a board is present to alert the athletes of their warnings.
Three warnings and the athlete will be shown a red paddle which indicates they are disqualified.