A forced write is a mechanism that forced the database to write data to the disk before it is committed to the database. This is useful because, in an event of a system crash or power failure, the data will not be lost.
Without Forced Writes, the process is minimally quicker, but when working on a Windows platform, Windows decides what should be saved to file, where and when, and the data pages are saved to file last.
The Firebird Forced Writes mechanism writes the data where it is needed. For instance, if a new data page needs to be opened in order to write data into it, it makes the necessary notes in the contents that this page contains data for the relevant table and also makes notes in the primary pointer pages for the table itself. Lastly, once everything has been successfully committed, a record of what has been done and that it has been committed is made in the TIP.
The main benefit of forced writes is increased security, by forcing the write to the disk before committing the transaction.