In a normal ”triangular” STP design, we have two possibilities regarding the port states on C:
So the port on C towards A is either RP or BL. Now what if the link between A and C becomes unidirectional such that C does not hear superior BPDUs from A? Then it would think it needs to send its own (inferior) BPDUs to A, because this port must be downstream right? This interface on switch C would therefore move to Designated state. When this happens, the dispute mechanism on A will block the link.
Why? In simple terms, this is like A saying to B: i’m richer because i have a million dollars. Now if B says to A: I’m richer because i have a thousand dollars. A knows that the only reason B is saying this is because B can’t hear A. The link is unidirectional and must be blocked.