<p>The history generator node creates a super long hashchain, also known as proof-of-history, which has to be done sequentially and thus on a single core/cpu, thus taking a long time to generate, and then publishes, say, every 500th hash from the hashchain. Then the verifiers hashes each of the published hashes 500 times and checks that the final hash from that matches the next published hash. This, unlike the proof-of-history generation, is trivially parallelizable, enabling the quick verification of the proof-of-history. The history generator can also append transactions to the published hashes before generating the next hash in the chain, enabling it to prove the sequence of transactions.</p>