The idea is to reduce the drag produced by the large open-wheeled slicks. It's a concept that many use on superspeedway ovals. Other design modifications center on suspension pieces and wings. Again, the boundaries are set by published rules, but there is room for creative thought and design. This is the period where most of the initial assessments are made.
Testing provides a baseline for mechanics and engineers.
Don't be seduced by the speeds and times you read in news releases from test sessions. They have little meaning when CART or the IRL convene at a track for a race. Between now and the race date teams massage, tinker and develop devices and parts that will give their car a unique signature. Just ask de Ferran. Jack Arute mans the pits for ABC Sports' auto racing coverage.