Spike nailed the mechanics pretty well there. Now it is possible for just leeches to complete a download - but only if the leeches have different but overlapping parts of the download. Like if you missing 5megs in the middle of a 100 meg dl - and I have that part - but am missing the last 5 megs - we can "leech" off each other and both get the full file - making us "seeders".
Not that any of that is really important
