In a way Spa shares many similarities with Montreal, where the track is divided up between long fast straights broken up by medium to slow speed corners. At Spa, you have a very fast first and final sector of the lap, with the long middle sector comprises of long medium speed corners. Therefore the car has to be setup to take maximum benefit of these fast sectors where minimising drag is a priority, but also take into account the long and twisty middle sector, where excellent aerodynamic performance will result in a good laptime.
Because of the nature of this track, many teams opt to run medium-to-low downforce rear wings and slightly less wing angle on the front wing itself to give good performance over the two fast sectors, but still having enough downforce to get away with the middle sector.
While HRT may not have the budget and/or infrastructure available to bring a raft of updates to many of the races, they still have updates that appear on the car every so often. The front wing was updated at the Turkish Grand Prix with a more prominent winglet section on the front wing, and this has also been updated for this Grand Prix.
Before, the winglet was more parallel to the main plane of the wing on the both its upper and lower element. This has now been changed to give the winglet a more curved appearance bringing the lower part of the winglet closer to the main plane of the front wing and also increasing the lower elements surface area. This is likely to redirect airflow better over the front wheels, reducing overall drag.
In a bid to reduce more drag due to the nature of the Spa-Francorchamps circuit, the two main elements of the front wing have been drastically reduced in size, to the point of there being a rather large void on the top of the front wing, which is highlighted in yellow. This is the easiest way to reduce drag without having to design a new front wing specifically for this race.
Williams also decided to race a different rear wing which first made its appearance at the Canadian Grand Prix in practice.
The rear wing itself may looking strikingly different to anything else that has been seen on the grid, but its application is much simpler than its aesthetics. The highlighted yellow section shows where the lower plane of the rear wing is usually placed on a medium downforce specific circuit. Due to Spa-Francorchamps nature, the wing has been curved upwards on the outer edges which will produce much less downforce, but still retains the depth of the normal wing in the middle of the lower plane, producing more downforce than the leading edges.
Williams obviously felt that the rear of the car requires additional downforce than what a smaller profile rear wing can produce for optimal balance around the lap; the rear wing seen on the Mercedes W02 car for example is clearly able to produce more overall downforce allowing for a smaller rear wing with less drag production. A Guerney flap is also present on the upper edge of the DRS element of the rear wing to produce additional downforce. This ran on both cars throughout the weekend.