Lets say Driver A is driving on the track and you plan to swap with Driver B
Driver A goes into the pits, stops at a box, and only until that pitstop is finished can you swap the drivers.
To swap, both drivers need to do a sort of an 'agreement' with each other. Press 'N' to bring up the connections list and beside the "SS" buttons you normally use to send setups, you should see a "T" I think it is. The person planning to switch should press it first, then Driver A that is in the pits should press his "T" (which is lit up) to 'accept' the switch. You (Driver B) should then appear in the car, and Driver A will be spectating . And off goes Driver B!!
Didn't get to test it, so the first time was in the race. Worked perfectly.
Swaps were very difficult in NR2003 so I'm happy to see this feature working so smoothly.