How does the Windows NT client (v4.2) open files for backing up?

Last night we had a situation that TSM was backing up a file and another
application wanted to open that file for read/writing and the second app
crashed. I thought TSM open the file so that it allows all other processes
to also open the file for either reading or writing. Why else detect that a
file has changed if this wasn't the case?

Thanks

/gjs