The photo in that thread doesn't depict the shark fins, but a different wing that BAR had been proposing, but had never raced. Someone other than myself probably has a better understanding of the progression of events, that led to the shark fins being declared "semi" illegal and cut down to minnow size, but I will give you what I know:
1) There had been some discussion regarding the "scarbs" wing that is featured in the other thread, which led to a rules clarification from the FIA. This wing was seemingly a method to get around the rules restricting the use of a third wing element.
2) The shark fins which had been raced in sepang and bahrain, (perhaps even melbourne) were affected by this rules clarification.
3) In san marino BAR truncated the shark fins so that they were attached to the wing at the foremost part of the wing, but cut short, leaving a gap to the rear wing edge. There is a close up of this on the atlasf1 main page that accompanies the article titled: HEAD: JAGUAR, NOT FERRARI CAUSED BAR'S REAR WING BAN
The rules clarification must specify that there be an empty space in the area around the rear wing edge.
Due to a technical clarification that sent to the teams on the Thursday prior to the San Marino Grand Prix, the fences on the BAR and Williams rear wings were deemed outside the regulations and were effectively banned.
BAR had adopted a fence on their rear wing that sought to segregate the three pressure regions created by the two curves in the rear wing shape. These fences kept the drag-inducing vortexes created at the wing/endplate interface to a minimum. This provided a clear performance benefit and the other teams had stated to the FIA that they would adopt these ungainly fences if the rules allowed them.
This year the rear wing regulations were revised to allow only two elements on the top of the rear wing, in the rulebook the FIA does not state what constitutes an element, simply using the term a “closed section” that doesn’t attempt to explain the shape or form of the wing.
This vague approach is a deliberate policy of the FIA, as it allows for later “clarifications” to be made to react to teams' interpretations of the basic rule. Often when teams have a solution that they feel is on the limit of what the rules allow they present it to the FIA technical delegate Charlie Whiting, who will approve or reject solutions based on whether he feels they are within the spirit of the regulations. Equally, when teams run a new solution in a Grand Prix, it can be protested by other teams or the FIA itself. Whiting’s interpretations of the rule often go back to the FIA's fundamental view of how an F1 car should look and work, hence more radical or unconventional solutions are often within the wording of the rules but are later placed outside the rules after a “clarification”.
In last week's clarification, the FIA has explained what it considers to constitute a wing element. According to Whiting, a wing element - which we would understand as an aerofoil profile - needs to be formed of two surfaces; one upper and one lower surface. This needs to be across the full width of the wing, and although the shape may change, the number of surfaces should be consistent.
While this clarification allows the complex 3D shapes to remain, it does not allow the shape of the 'fence' used by BAR and lately Williams and McLaren, because the 'shark fin' shape consists of one more surface than the rules allow (i.e. three in total). BAR's response was to change the shape of the fence to remove the sharp (two part) upper edge and create a single line curve, which still provides some of the fence effect while meeting the regulatory profile.
In Testing, BAR had also tried a wing with multiple (20) fences that would have enhance the effect even further. While the rear used in the Silverstone test also sported a third element, it is not clear if this was to hide the multiple fences or was a part of the design to effectively create an extra element by adding it to the rear flap.
