Hashfunctions have this funny feature called "preimage resistance". It means when you know the hashfunction and the hash you cannot calculate the inputs into the hashfunction.

This is used in any Lightning transaction. The sender, recipient and intermediate nodes first negotiate all transactions but to make sure the all go hot/valid simultaneously the hash preimage is released only once

https://docs.lightning.engineering/the-lightning-network/multihop-payments/hash-time-lock-contract-htlc