Tools/Options/Wipe
QS uses the excellent open-source file wiping software, Eraser. You won't find a better product. Temporary files are created on disk during the encryption process. Only encrypted data is saved to disk. As soon as they are read in, they're removed. Essentially, wiping overwrites the complete file multiple times with random data. Recovery of the data, if possible at all, requires extremely expensive equipment and significant time to achieve very limited results.
Eraser Options
As noted above, only encrypted data is saved to disk in temporary files during the encryption process. Obviously, this data is already secure so it's up to you as to whether you actually want to wipe the data (as opposed to just delete) and how thoroughly you want to wipe it. Disabling this option tells QS not to use Eraser but instead just delete the files. For some, deleting is a satisfactory method of data removal.
Important Note
The overriding premise of this version of QS is that it keep all data in its home directory and that it make no registry entries. Eraser is open-source and I've modified it to eliminate registry calls. You must not replace this with an eraser.dll from another source. In fact, QS won't allow you to replace it.