I am unable to understand how to use the locked timetable. Is it possible to lock a teacher's timetable? In what ways can locking help the solving process?
in normal case it never helps to lock larger parts of the timetable. you don't need it for generating good timetables. in normal case you won't get good timetables with partly locked timetables.
locking is in fact a feature request from timetablemakers that try to keep the old timetable as unchanged as possible, because:
1) they have already released the timetable, but they sadly done a bug
2) there are other minor changes needed (maybe a teacher is suddenly not possible to work as much as before and you need to change the activity planning / timetable) and they try to keep the timetable as unchanged as possible
i highly recommend to read the following 2 chapters before you try to use the locking feature to fix bugs:
https://www.timetabling.de/manual/FET-manual.en.html#id_39