The aerobody device sits on the stem, and measures relative position of the rider’s head, chest, and waist using a non-contact optical sensor.  This gives you a measure of rider position which is accurate to around 1mm, allowing the cyclist to check that they are maintaining their optimal position throughout their ride, be it on the road, in the velodrome or even in a wind tunnel.  The device can also be used on static trainers, allowing the rider to train in their best aero position.