The Garmin Tri Heart Rate Transmitter for 920XT and the Fenix 3 has been specifically designed with triathletes in mind. Enhance your triathlon experience by pairing HRM-Tri with your training computer or watch (please check compatibility). The HRM-Tri stores heart rate data when underwater, then forwards it to you compatible watch. Garmin's smallest and lightest module fits within the width of the strap. Its soft, rounded edges and covered seams make the strap super comfortable out of the water, too. The Transmitter reads 3 metrics; Cadence, vertical Oscillation and Ground contact time. These 3 metrics, once viewed after you activity, will help you train more specifically and on areas that aren't your strongest. Wearing triathlon kit is a must when using the Tri Heart Rate Transmitter as it keeps the monitor close to your chest and in one position, keeping the precise.