BitTorrent fully supports stopping and later resuming a partial download. You don't have to do anything special. If you cancel a download before it's finished, the partial download remains on your hard drive. To resume the transfer, just click on the same torrent link again and when asked where to save the file, select the same location as last time. BitTorrent will see that the file exists and check it to see how much has already been downloaded. It will then pick up where it left off the last time. See also the section Why is my downloaded file huge even though I only downloaded a small bit? regarding file size.

Note: To resume properly, you must make the same selection when prompted as you made the first time. For torrents consisting of a single file, this is rather straight-forward: simply select the file. However, torrents that consist of a folder of multiple files can be a bit more confusing. To resume, you must select the folder that contains the BitTorrent folder.

Here's an example of resuming a folder-type torrent. Let's suppose that you downloaded a torrent called SomeCoolBand, and selected to put it in the folder Downloads. So your directory structure resembles something like \Downloads\SomeCoolBand\file1, \Downloads\SomeCoolBand\file2, and so on. The important part of this example is that should you resume this transfer, when asked to select a destination folder you must select the \Downloads folder and NOT \Downloads\SomeCoolBand. It may seem a bit counter-intuitive, but just remember to always make the same selection as the original choice. When you first started the transfer there was no SomeCoolBand folder; you instead selected \Downloads and BT created the SomeCoolBand folder.