Complex sentence: In Windows, defragmenting (on FAT file systems, at least) after deletion may overwrite the original position of the file, making recovery more difficult, but is not a recommended way to ensure secure data deletion.
Split into simpler sentences: In Windows, ordering a defragmentation (on FAT file systems, at least) after deletion would possibly overwrite the original position of the file, making recovery more difficult. Defragmentation is, however, not a recommended way to ensure secure data deletion.