Lightning network consensus mechanism for payments that run across multiple nodes

In the HTLC protocol of the Lightning network, there is a transaction in which the money is routed from A via B to C. C creates a secret and generates the hash of it with SHA-256, then C passes the hash on to A. A passes this hash on to B. Now B must reveal the secret to authorize the payment to B, and B then passes the hash to C. C must then reveal the secret in order to receive the payment. But how can the secret be presented if only C knows the initial value? Is the initial value passed on to every participant, but not to A?

I'm having trouble understanding the consensus mechanism of the Lightning Network. What are these "secrets" and how do the Hashed Time Lock Contracts work? How are they fulfilled? By guessing the hash value, but that would again consume energy and time, which is rather contrary to the basic idea of the Lightning Network.