The other thing about making it obvious you're looking back is that drivers are (or should be, which obvs isn't quite the same thing) trained to interpret a cyclist looking back as a cyclist who is about to do something.
It often shows up as a segment in the hazard perception video during theory tests as something that should make drivers pay attention. Anecdotal experience on fast A roads in particular is that if I look behind a lot, drivers are more likely to change lane when they overtake me. No looking behind, many more close passes. The difference might be that when looking behind they almost subconsciously think "oh, a hazard" and make an effort to move away.
The other thing about making it obvious you're looking back is that drivers are (or should be, which obvs isn't quite the same thing) trained to interpret a cyclist looking back as a cyclist who is about to do something.
It often shows up as a segment in the hazard perception video during theory tests as something that should make drivers pay attention. Anecdotal experience on fast A roads in particular is that if I look behind a lot, drivers are more likely to change lane when they overtake me. No looking behind, many more close passes. The difference might be that when looking behind they almost subconsciously think "oh, a hazard" and make an effort to move away.