This is such an objective issue. Ask any race steward who has been forced to make a call on some situation which ended in tears. Sometimes 'no call' seems to be a correct evaluation.
Many worthy viewpoints precede mine here. I prefer to use the conceptual terms 'proactive' & 'reactive' to assess who primarily is responsible for fault in an alleged conflict. In braking zones, especially where the speed delta (of trap speed to corner entry speed) is great, I like to believe a driver has one chance to establish a primary line & then one chance to choose an alternative line. This leading driver, however, must make these moves not in hasty response to the progress of a trailing driver. Make a bed & lie in it, as it were. More lateral movement than this is acceptable only in response to unexpected traffic obstructions directly ahead, that is, collision avoidance. Furthermore, I believe that the driver ahead has priority of line choice above those driving behind, even if only two thirds car length behind.
We must recognize the fourth dimension, namely time. Others have mentioned concepts like brake checking & delayed acceleration. This temporal consideration comes into play in varied forms. I am fond of using subtle adjustments of racing rhythm cadence to control the pace of followers. Similarly, lateral movements may be slower resembling fades rather than quick crab walks to an alternative lane. Keep 'em wonderin', I say. These tactics also can be useful in tire/fuel preservation. They also can provide a tenth or two of grace lead to hold another driver at bay for another turn or two. Excessive, deliberate fluctuations of flow are neither warranted nor welcome.
Remember that these conflicts occur in many areas of the track, not just braking zones. Motion on longer straights can be useful in draft dodging. I recognize that this is not an instance of blocking introduced in the OP, but this exaggerated motion can fall under of the blanket of dirty driving. Also, on a grid start some car/driver combinations have an advantage. Line alteration here can prevent a 4WD vehicle from gaining a position early.
Now for my disclaimer section. I (in my humble opinion
) am not a dirty driver. I also, at C/S rating with merely two dozen races under my belt, am not an alien. I retired when I felt I had reached my reasonable potential (can't quit the day job) & when I disliked behavior of others which robbed me of my ability righteously to excel above them. Popeye The Sailor famously claimed, "I am what I am & that's all what I am." I'm quick enough to realize & to recognize when another is better. I don't just roll over, but I mostly operate under the principle that if I can't earn a position with my driving ability, I give way with no more than reasonable resistance. Viewed selfishly, I feel it is in my best interest to let a faster driver get on his way because this loses me less aggregate time, even if it costs me one position. If I permit an A driver through easily, the three B & C drivers still behind me aren't up my tailpipe as they would be if I give the A driver an unduly hard time. He's passing me either way, right? I firmly do not subscribe to the notion that rubbin's racin'. I give room where it is available. Sometimes I 🤬 up a corner. Fine, my bad, go on by. If I can't hold someone back or get by someone without supplying the 'chrome horn', then I yield.
Conversely, I'm not the slowest bee in the hive either. I appreciate clean driving from those few who are less skilled than I am which provides me a chance to pass where I'm clearly better.
Thx for the discussion. I look forward to more opinions.