When Eaton needs to capture crucial intel in real time, he uses a $120- to $500-a-year subscription-based app called Coach’s Eye. It lets his trainer record his moves on an iPhone, overlay voice notes, draw on freeze-frame images, or even measure the angle of, say, Eaton’s elbow. It also has a scrubbing feature—the ability to scan in slow motion, backward and forward, and to examine micro movements. “We tend to break movements into three phases: the start, middle, and finish,” says Eaton. But even within those movements, his coach breaks it down into smaller phases. “So it’s great being able to drag your finger on the screen and really see in each frame a more technical aspect of what you’re doing,” he says.