This is about the horizon which divides us from stuff that is too far to see because it's moving away from us faster than the speed of light.
If point $A$ and $B$ are so far away that $B$ is a bit outside $A$'s horizon, what if I look at the point $C$ that's midway between them? It will get light from $B$, because there's nothing from stopping that happening. However, any light at $C$ must be moving with speed $c$ with respect to $A$ while $C$ itself is moving slower. Therefore, any light that is at $C$ should also reach $A$ if the direction is right (and it is, if it came from $B$).
Where is the flaw?