New rules for overtaking and blocking.
A driver may not deliberately leave the track without justifiable reason. More than one change of direction to defend a position is not permitted.
Any driver moving back towards the racing line, having earlier defended his position off-line, should leave at least one car width between his own car and the edge of the track on the approach to the corner.’
Sounds clear. Early speculation is that the interpretation of "one car width" might be contentious. Also, a driver might drift slightly off the line and claim he wasn't blocking.
Personally, I like this. Gives the lead driver a chance to block but forces him to time and place his one blocking move very carefully.