Deleting files using the recycle bin is not secure because the operating system does not actually remove the file from disk. The shred files feature overwrites selected files in an attempt to prevent them from being recovered.
The shred files and shred folders features can be accessed by going to Tools => Shred Files or Tools => Shred Folder. Then select files or a folder to shred.
NOTE: The developer of Kryptor is NOT responsible in any way if you use shred files to overwrite files you didn't want to overwrite. Do NOT select files/folders that you do not want to delete.
Shredding a file causes the contents of the file to be overwritten in a certain way specified by the chosen 'Shred Files Method' in settings. The default setting is '1 Pass' (see below).
After a file has been overwritten, Kryptor anonymously renames the file, replaces certain metadata about the file (creation time and last write time), and then deletes the file.
Here is a breakdown of the different 'Shred Files Methods' in settings:
Note that although using shred files is better than regularly deleting files, it is still not necessarily going to prevent data recovery. There are various filesystem limitations that mean this method of erasing files is nowhere near as effective as wiping your hard disk.
If you want to ensure that files are completely removed from disk, either wipe your hard disk with one or more passes of pseudorandom data, encrypt your drive (ideally after wiping it), or physically destroy the drive.