The diffuser on the Brawn car is helping them set the pace in F1

By Mike Gascoyne BBC Sport pundit and former F1 technical director

A new word has entered the sporting lexicon over the last few weeks thanks to a typically complex row that has split Formula 1 down the middle.

News bulletins have been led by items about diffusers - a critical part of the design of a Grand Prix car, but one that was largely unknown outside an F1 paddock until the last few weeks.

Aerodynamics are crucial to the performance of an F1 car - they define how much downforce it creates, and therefore how fast it goes.

And the diffuser, which is at the back of the car between the rear wheels, is one of the most important single parts in creating that downforce.

An argument about the diffusers on the cars used by Brawn Grand Prix, Toyota and Williams ended up in F1's Court of Appeal in Paris this week - where judges ruled their controversial 'double-decker' design legal.

Here, I will explain why the issue is so important - and how it could easily decide the destiny of this year's world championship.

It is very difficult to make the F1 rules watertight and the diffuser row has proved just how dramatic the results of that can be.

When the sport's governing body, the FIA, decided to shake up F1 and make racing more exciting by increasing overtaking, the guidelines on diffusers were open to interpretation.

Renault have produced a revised diffuser in time for the Chinese GP

After all, the FIA has two or three people making the rules whereas each team has about 150 people working out how to exploit them.

Ross Brawn, whose Brawn Grand Prix car is leading the field with its 'double-decker' diffuser, warned rival teams last season that the rules governing the rear of the car were open to interpretation.

But, as you would expect, some of the teams who had already started development for 2009 - including Renault and BMW Sauber - did not want to change the rules to make them clearer.

So the teams' designers and aerodynamicists began looking for loopholes and ways to exploit the regulations.

If a team wants to add something to their car which they are not sure is allowed, they will ask the FIA for clarification.

But whether you get a yes or no from the powers that be often depends on how you phrase the question.

That could explain why Brawn GP, Toyota and Williams went ahead with their split-level diffuser design and the other seven teams did not.

The teams say the innovative design is worth half a second per lap - that is a massive step in F1 Mike Gascoyne

No matter what the intended concept of the 2009 cars was, the rules as they were written did not get it right.

The Court of Appeal's decision to reject the protest against the double-decker design was correct because it was based on a black-and-white interpretation of those rules.

The rules say that the height of the diffuser must be limited to 175mm above the floor of the car and that there must be no bodywork above it.

Brawn GP, Toyota and Williams have exploited a loophole by incorporating the crash structure, which absorbs the rear impact in the event of an accident, into the diffuser design.

The teams have shaped the central part of the crash structure to allow air to flow through it, creating more suction and downforce.

That is why the design is called a double-decker because below the crash structure at a shallower angle is the regular diffuser which complies with the height restrictions.

The Red Bull team will find it tough to fit a new diffuser to their car

The secret is that that part is aligned with the air flow and so does nothing, it is actually the top tier of the crash structure that is acting as the diffuser.

The air reaches the higher part of the diffuser design through holes in the stepped floor of the car.

The central part of the car - referred to in the rules as the reference plane - is 50mm lower than the floor on either side of it - the so-called step plane.

The holes are where the reference plane meets the step plane and just before the point where the floor starts to sweep upwards at the beginning of the diffuser.

The majority of the teams thought this area had to be closed off without any holes but Brawn GP, Toyota and Williams thought differently.

The rules governing this area were written many years ago and were not meant to deal with this situation - so as they are written they allow the holes and the split-level diffuser to exist.

The teams say the innovative design is worth half a second per lap and that is about a 5% increase in downforce.

That is a massive step in F1. To make that leap, technical teams would have to spend thousands of hours in the wind tunnel. So for a diffuser - just one part of the car - to provide that much gain is huge.

You can see the advantage translated into real results - in the first two races in Australia and Malaysia, five of the top eight finishers were running with the split-level diffuser.

The problem teams have in reacting quickly is that they will not only have to change their diffuser, they may also have to redesign their gearbox and rear crash structure.

A new crash structure means new crash tests, which can be quite a long process. For some teams, this could mean one or two months instead of one or two weeks.

Toyota and Williams launched their cars in January and that was the first chance for their rivals to get a good look at the diffuser design.

The Brawn's diffuser cleverly exploits a loophole in the F1 rules

If they decided to react then, they will already be quite a long way down the development road.

For example, Renault may have been arguing in court about the diffuser on Tuesday but they have produced their own upgrade for use in China this weekend.

If teams were predicting the FIA would ban the design, then it could easily be up to two months before they have the new parts on their car.

Ferrari, in particular, are struggling on this front and do not expect to implement any changes until the Turkish Grand Prix on 7 June.

Red Bull will also struggle because of their unique gearbox and pull-rod suspension design. They will need a major update and designer Adrian Newey is working around the clock to make the changes.

I believe most teams will react quickly but will initially only gain two or three tenths of a second instead of the full half a second - but that extra time is still worth having.

I think it could. The championship will begin to tighten up but any points garnered now could be the difference between this season's winners and losers.

The key for Brawn GP, Toyota and Williams is to make hay before the seven teams catch them up - because they will.

Jenson Button and Brawn have two race wins under their belt and will expect to maintain their advantage in China, Bahrain and Barcelona.

Button could build up a 30-point lead and that may be enough to see him through to the title.

Toyota have a couple of podiums but may have wanted more while Williams will be disappointed as they have not capitalised on some very quick performances from their car.

I've always loved a change of rules because it is an opportunity to use your skills as an aerodynamicist and make a real difference.

I take my hat off to those designers who have not been blinkered and thought outside the box - well done and I wish it was me!

606: DEBATE Renault and McLaren will be the first teams with an updated diffuser - surely they can get closer to the front now hammydigrassi

Hopefully, I would have been at a team who would have come up with the split-level diffuser design, or at the very least would have reacted quickly to it.

I'm surprised that three months on from the Toyota and Williams launches some teams are still being slow to respond.

I would have wanted the diffuser on my car in Melbourne - anything else would have been a huge disappointment.

Mike Gascoyne is working as a pundit for the BBC at the Chinese Grand Prix. He is a former technical director of the Force India, Toyota, Renault and Jordan teams. He was talking to Sarah Holt.

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