Users can save a file before a miner seals it into a sector thanks to having deals on chain. The details can be found in The Filecoin Spec (https://github.com/filecoin-project/specs). The miner can then seal the sector when it is filled with deals and users can retrieve the file from the sector later. Note that users can also retrieve their file immediately before it is sealed into a sector. This is not currently implemented but we plan on doing this soon.