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Leeching is when your client downloads but doesn't allow uploading.
It is important to notice that it becomes a leech when it doesn't allow. This means that if you download from someone but you are not uploading to him at that very moment you aren't leeching as long as you allow him to download at some moment (when that client gets to the top of the queue).
Such clients are banned as soon as they are detected by other clients and they are called leechers or bad guys.