In cycle racing the major resistive force acting on the bike & rider is aerodynamic drag. Other forces include rolling resistance and the effect of gravity, which become more important as the steepness of the road increases.
We measure aerodynamic drag in CdA, which consists of two factors: your frontal area (A, measured in m 2) as well as the drag coefficient of the shape (Cd, a dimensionless value). These two combine to give a CdA in m2. The lower your CdA the more aerodynamic you are, and the faster you can ride with the same power output.
How much difference does it make?
A small reduction in aerodynamic drag can have a large impact on your race performance - for example a rider completing a 25 mile/40km time trial in 1hr will reduce their finishing time by around 30sec for just a 2% reduction in their CdA.
Longer events such as Ironman triathlon bike legs (112 miles/180km) can see many minutes worth of savings with better positioning and equipment.
In AeroCoach sessions typical CdA reductions are between 5-15%, resulting in large performance improvements of 15-35w .