Because of a considerable simplification of upper bodywork, the aerodynamicists at Williams are spending a lot of time to investigate the efficiency of the car's underbody. It is well known the this part of the car can bring a lot of downforce with a low drag penalty. For the same reason, the team developed their double step diffuser which reportedly is good for 15% more downforce compared to the simple designs that follow 'the spirit of the rules'.
More at the front though, at the sidepod's entry, the outer edge of the car's floor curves up. In this area, the airflow is from the inside of the car towards the outside due to the splitter underneath the cockpit. To guide this airflow, the upward edge of the floor effectively creates a venturi which reduces pressure under that section of the floor and hence creates downforce. The 3 vertical elements under that curve help straighten the airflow similar to the elements attached under the frontwing or in the diffuser.