1. What's Multi-MFT file?
   In general, NTFS save one file's attributes in one MFT record.But in some case, a file has so many attributes that can't be saved in one MFT record, several MFT records maybe used. More information about NTFS Files and Data Storage, please see:
   

   http://www.pcguide.com/ref/hdd/file/ntfs/files_Files.htm




2. When does Multi-MFT will be used?
   When a file is huge and fragmentation is heavy,Multi-MFT will be used to store file attributes.



3. Why is it difficult to undelete Multi-MFT file?
   When Multi-MFT file is deleted, all MFT records is cleared, except the first one.That's means: You can't get enough information from MFT records to undelete files.So extra work must be done to get those information.

   NTFS File Rescuer is the best tool to recover Multi-MFT files!

Usefull links:
http://forum.sysinternals.com/forum_posts.asp?TID=11539