WRC cars often travel over 100 mph on courses that can be paved, gravel (dirt), even snow. The races are run in all types of weather, including rain, so the course can be extremely slippery. And these are not race tracks, these are real roads, usually backroads in the countryside, so you can imagine what the conditions are like.
Each team goes through the course one at a time, a few minutes apart. Each one is timed, and the fastest time wins the stage. A single event includes anywhere from 16 to 27 stages, over the course of three days. The fastest total time over all stages wins the event.
There's a driver (duh), who keeps the car on the road while going as fast as possible through the course.
This is where the co-driver comes in. The co-driver is a navigator, in a sense. By reading the pace notes, the co-driver is alerting the driver so that he can anticipate what's coming up and brake early enough for a tight turn, or accelerate going into a straightaway before it's entirely visible. The co-driver has to tell the driver what's coming up early enough so that it can be anticipated and speed can be maximized, but not so early that the driver has to remember too much. It's a delicate balance, and really good rally teams have almost a mind-meld, such that the driver is fully committed to what he is told by the co-driver, often in spite of what he sees.
As for the notes themselves, there is a short-hand system of notation that is used to capture every feature on a recon run through the course. The notes are then read out in a somewhat expanded form by the co-driver. When a team is really successful, like Loeb and Elena, it's a pretty amazing thing to watch.
The notes look something like this:
MC1 100 KL2 100 KR2 200 SQL 100 KR4 50Y!->R2+ (D/C!) 100 +SQR 400 F->CR->KL4 100 MC2
Which translates to this:
- From Main Control 1 (start), 100 metres straight to a kink left, severity 2
- 100 metres, kink right severity 2
- 200 m, square left (90°)
- 100 m, kink right severity 4
- 50 m, yump (caution!) into immediate right hand bend severity 2 tightens (caution, don't cut [the corner, due to hazard on the inside]!)
- 100 m, oversquare right
- 400 m, flat (maximum speed) into crest into kink left severity 4
- 100 m to Main Control 2 (finish)
At first, it can be hard to see the value of a co-driver. I mean, where else do race car drivers have a copilot? But when you think about the speeds and the terrain they're travelling over, you can start to appreciate how it would be nearly impossible to make it all the way through a course, with no practice runs, often at over 100 mph, without what amounts to a mental map of what's coming up in the next 10 seconds.
If you were driving 80 mph on a slippery dirt road and there was a hairpin turn coming up and then 100 meters later a ridge (that's a jump at those speeds) and right after the ridge a sweeping turn in the other direction, you'd probably want to know that ahead of time.
In the last few events (Finland and Japan) you can see some examples of the co-driver messing up the pace notes. In one case, it almost reduced the co-driver to tears. Pretty intense stuff. So go watch some rally and hopefully you'll have a new appreciation of the art of the sport.