Basically, to shoot someone in the back there needs to be no single point of your movement that is: 1) Able to trace a direct line from Silhouette to Silhouette unimpeded by terrain. AND 2) In the Front Arc of the enemy, disregarding actual terrain. If only one of these is true, then LoF is not established.
If you never angled to shoot someone in the rear facing by cutting the pie even though you were attacking them from their front arc. Functionally this change doesn't affect you. The big take away is that a trooper facing a wall can aro to people shooting him no matter which way they come around now.
Though when they're facing a wall they have such narrow sight lanes it can be possible to Cautious Move into their back arc for some nice free shots! Obviously you need Stealth to do this within their ZoC.
Perhaps I'm misunderstanding, but if the only point that the silhouettes can see each other is when you are in the back arc of the reactive trooper, then they will not be able to shoot back, no matter whether you started your movement in their front arc. This is the same reasoning that doesn't allow you to shoot someone entering base contact from your back arc unless you have 6S. By the time you are able to react, you do not have the option to BS attack.
Emphasis mine. Being completely in someone's back arc is the only way to shoot them in the back and avoid an ARO. Overall I like the ruling. It does get a little non-intuitive sometimes, but it helps that the drawing explicitly shows that all you need to be is in someone's front arc for them to ARO you. It's also very quick to judge.
