Forward recovery "replays" all database updates in forward direction till the specified point in the journal file. Forward recovery on a backup database starts from when the backup was taken and continues till the specified point in the journal files. Forward recovery on an empty database starts from the beginning of the journal files. 

Suppose a system crash occurred at 08:50 hrs and a backup of the database was taken at 08:26 hrs. Using forward recovery, you can replay the database updates between 08:26 hrs to 8:50 hrs (in blue) on the backup copy of the database and restore the database to a state prior to the crash.  In the process you can also identify unfinished or broken transactions that might have occurred at the time of the crash. In the following illustration, X denotes the crash time and the blue updates denote forward processing.

A command like mupip journal -recover -forward -before="--8:50" gtm.mjl performs this operation. From the current journal file, forward recovery moves back to the point where the begin transaction number of a journal file matches the current transaction number of the active database (the point when the backup was taken)  and begins forward processing. Since a journal file is back-linked to its predecessor, GT.M facilitates forward processing by activating temporary forward links between journal files that appear only during recovery. These forward links are temporary because they are expensive to maintain as new journal files are created. Note: Forward recovery, by design, begins from a journal file whose "Begin Transaction" matches the "Current Transaction" of the active database. This condition occurs only when a new journal file is created (switched) immediately after a backup.  If a database is backed up with MUPIP BACKUP -NONEWJNLFILES (a backup option where journal files are not switched), forward recovery cannot find a journal file whose Begin Transaction matches the Current Transaction and therefore cannot proceed with forward recovery. Always use a backup option that switches a journal file or switch journal files explicitly after a backup.  Also, once a database has been recovered using forward recovery, you can no longer use it for a future recovery unless you restore the database again from the backup.