Cal. Code Regs. tit. 4 § 1693
The horses are under the control of the starter from the time they enter the track until dispatched at the start of the race. After entry on the track or race course, the horses are not entitled to further care from their attendants, except in case of accident the starter may permit the jockeys to dismount and the horses to be cared for during the delay; otherwise, no jockey shall dismount until after the finish of the race. The horse must be started by the jockey, but with the sanction of the starter it may be led to its position in the gate by an assistant starter. With the sanction of the starter an assistant starter may enter the gate to handle a fractious horse. No assistant starter shall in any way impede, whether intentionally or otherwise, a fair start.
Cal. Code Regs. Tit. 4, § 1693