Technically the incomplete files are shared the second they're created (assuming the user has allowed the sharing of partial files and the queryhit has given us a hash of the complete file). For zero-length (and other small files) this allows alternate locations to be exchanged. Realistically, you will only receive requests for the file after approximately 25% of it is downloaded (or 5MB, whichever is larger). This is because you will not insert yourself into the 'download mesh' until that threshold is reached. We're looking at tweaking those numbers.

Not all files in the incomplete directory are shared -- only those with an associated entry in downloads.dat. This is because we limit partial file sharing to requests by hash in an effort to reduce corruption. You may also notice that if, for whatever reason, the "Your download is corrupted" message appears while downloading, then the incomplete file will be unshared (and hopefully turn back to red).