What is a passkey?
Your passkey is a random set of 32 characters, guaranteed to be unique to you. It's stored in your member Profile (visible in USER/CP) and can be used to discover your identity. With the passkey, the Tracker Server does not need to use your IP address to figure out who you are. The *.torrent file itself contains your identity.
A copy of your passkey is automatically added to every torrent file that you download from the tracker. (By "torrent file" we mean the *.torrent file, not the data/video files which the torrent refers to.) When you use a *.torrent file which contains your passkey, the upload and download statistics are recorded to your account.
Why is keeping my passkey secure so important??
If someone else obtains a *.torrent file containing your passkey, or just your passkey itself, they can use your passkey to download content without having it recorded against their account. Instead, their downloading gets charged to your account. This can have a serious impact on your ratio, and if it continues long enough it may result in your account being disabled or banned. Therefore, it is very important to protect your passkey and to make sure that it isn't revealed to anyone.
Perhaps the most common mistake is to send someone a *.torrent file. This should never be done. Instead, send them the URL of the torrent's description page, and from there they can access the torrent (which will then contain their own passkey).
Never upload your XXXXX *.torrent files to another tracker, and never store your *.torrent files somewhere where others might find them.
Don't store your *.torrent files in a folder which you intend to share as a torrent. (Believe it or not, someone actually did this!)
Don't use Azureus on any site that doesn't have the private flag set. Azureus allows people to download a torrent from you over DHT (including your passkey).
These are not necessarily the only means by which someone could obtain your passkey. There are other methods which may be outside of your control. If you see torrents being leeched on your account which you know you are not downloading yourself, it's likely that someone has somehow come into possession of your passkey.
If you suspect that your passkey has been compromised, you should reset it immediately. Go to USER/CP and select "Reset passkey". Once you change your passkey, you will need to completely stop and start all torrents you were in the process of downloading from the tracker, redownloading new *.torrent files as necessary. These new connections (and new *.torrent downloads) will contain your new passkey. Changing your passkey will not affect any of your completed data/video files.
Note: You can only change your passkey if all active torrents have been stopped. If someone else is using your passkey, you might have to wait until they have stopped the torrents they are downloading. That might be a long time, hence the importance of making sure that your passkey is kept secret.
If you are unable to change your passkey because someone else is actively using it for data transfer, contact the staff for assistance. Visiting the XXXXX help channel on IRC (XXXXXX) is usually the fastest way to get this problem fixed. They can assist you in getting the offender disconnected from your account so that you can reset your passkey. Do not allow this problem to continue unresolved, because allowing someone else to download using your passkey could have a serious impact on your ratio and your account.