alright, the way this works is simple: When someone is slipstreaming behind you on the straight and is about to overtake it is not accepted to simply block him/her. However, one often sees race drivers nudge their car a fraction to the left or right (saying you're being overtaked from the right), but this movement is limited in so much that the driver overtaking isn't blocked but might be startled. This sounds weird, but go to a DTM race and look at cars overtaking on the finish line straight and this happens a lot. Hakkinnen once explained that this behavior is there to intimidate the competitor and, partly, because there is an upcoming corner (also to the right) and that perhaps the other driver would hesitate and hence abort his overtaking action. Again, blocking is not allowed, if someone passes you and there isn't an upcoming corner coming, but a finishline you are not allowed to block!
this is a statement of the FIA made in 1999
The 1999 FIA Formula One World Championship Sporting Regulations cover overtaking under "incidents":
"Incident means any occurrence or series of occurrences involving one or more drivers, or any action by any driver, which is reported to the stewards by the race director (or noted by the stewards and referred to the race director for investigation) which:
- caused an avoidable collision;
- forced a driver off the track;
- illegitimately prevented a legitimate overtaking manoeuvre by a driver;
- illegitimately impeded another driver during overtaking.
The stewards may impose a 10 second time penalty on any driver involved in an Incident."
This quite clearly bans using physical contact to overtake and prohibits blocking a driver attempting to overtake.
http://atlasf1.autosport.com/99/san/tytler.html
so, it's as simple as that!