WRC teams are allowed Recce. THis is where the teams are allowed a certain number of passes over the stages to create thier "pace notes". Pace notes are direct information as you can read in the above posts. They are generated by hte driver AND codriver during recce. This is VERY important for WRC, as one missed rock or kicker, and you have what happened to Markko Maritn(Ford Focus) at the last rally, where they were flat out in 6th(about ~130 mph) and cartwheeled because they missed something in recce.
Here in the US, we are allowed "Stage Notes". These are generated by a single, unbiased crew a couple weeks before the event, using GPS & G meters & cool stuff. It measures the geometry of the each turn & the crew notes all nasty objects & jumps & whatnot. We purchase these stage notes at the time of registration. They are the same format for all competitors in the rally, and the same format for every US SCCA ProRally.
The Codriver must not only read these notes in varied speeds (to give drivers like me time to react to the corners coming up), but also emphasize the really scary stuff, like triple cautions & things like "300 into L5+ > L2-" which means "300meters straight(flat out) in to a 4th gear Left(pretty darn fast), that CLOSES to SUPER SHARP Left(almost a switchback, slow 2nd gear-ish)".
All of that while I throw the car all over the road, bouncing & banging about. He cant call it too early, cuz then I get lost in his directions & forget it or get confused, and he cant call it too late, cua then we'd be off in the woods(or worse).
Add to his task list running the rally computer/odo/clock to get us to each special stage on the correct minute, also add getting us in/out of services on the right minutes, and also confirming our scores/times & our correct place in running order throughout the rally.
And then they also push the car back over when we roll, help change spare tires & repair stuff on stage, keep us calm after a roll or "off"(that's when you go off the road & may or may not wreck), answer questions like "how long to the end of the stage?" when we drivers need to make decisions on letting someone faster by or to drive out a flat.
Famous quote: "A CoDriver cant win a rally, but they can lose it."
Codrivers do ALOT, and are an essential part of the team. I am glad I have a good one.
JC