If your webserver is behind an HTTP proxy, it will look as if all comments and trackback pings are coming from the same IP address, making it difficult (nay, impossible) to ban any particular sender's IP address. If TransparentProxyIPs is 1, Movable Type will ignore the first IP address it sees and try to determine the external IP from which a comment or trackback ping was received. Note that, because there could be other HTTP proxies in the chain, this is still not necessarily the IP address of the machine that sent the comment or trackback ping. Only use this option if you know that your webserver is always accessed through a proxy, since otherwise the IP address information of the remote host will be lost.