When you start a fitness activity, your RECKON watch automatically increases its heart rate sampling to 25 Hz. This means the heart rate sensor is collecting data 25 times per second, allowing the watch to respond more quickly to changes in your heart rate during exercise.
When you end the activity, the watch returns to the heart rate sampling setting selected under All Settings → Sensors → Heart Rate.
For the best results, wear your watch snugly just above your wrist bone so the sensor maintains consistent contact with your skin.