Whereas Proof of Replication is run once to prove that a miner stored a physically unique copy of the data at the time the sector was sealed, Proof of Spacetime (PoSt) is run repeatedly to prove that they are continuing to dedicate storage space to that same data over time.
PoSt builds upon several elements created during Proof of Replication: the replica, the privately saved CommRLast, and the publicly known CommR.
First, PoSt randomly selects some leaf nodes of the encoded replica and runs merkle inclusion proofs on them to show that the miner has the specific bytes that should be there. Then, the miner uses the privately stored CommRLast to prove (without revealing its value) that they know of a root for the replica which both agrees with the inclusion proofs and can be used to derive the publicly-known CommR.
The final stage of PoSt compresses these proofs into a single zk-SNARK. (We’ll learn about zk-SNARKs in the next lesson.)
When miners agree to store data for a client, they're required to put down collateral. If they fail a Proof of Spacetime at any point during the contract, they'll be penalized. This is a key part of the incentivization that encourages good behavior amongst all the players in the Filecoin network.