Just as a hunter can scan woodlands and spot movement indicating his prey, Nagle can scan a line of horses in a starting gate and instantly know when a horse is acting up. It's not nearly so simple as it sounds.
Some horses refuse to enter the gate.
Others try to back out or kick their hind legs. Jockeys have been pitched forward or thrown backward, breaking arms and legs. Nagle must open the gate the instant every horse is in place and ready to go. The longer it takes, the greater the stress on the field, and the greater the risk the race will be decided by the quality of the start rather than the horses' breeding or the jockeys' talent. For the Derby, the number swells to 26, some of them volunteers from other states.