The ideal location for the Torso sensor is the center of sternum. However, a sensor can be attached anywhere along the sternum (from the top to the bottom) as well to make the attachment more comfortable. Attaching the sensor on the abdomen is not recommended.
The Torso sensor can also be attached on a heart rate monitor strap. However, please be sure to tighten the strap on the body otherwise the sensor may move in unexpected ways and may interfere with data collection.