No, it is always enabled. But it will not react super-fast, so please set an upload limit of about 75-85% of your upload capacity. (i.e. when you surf the web you use only little traffic for a very short time, so that UBT won't throttle the traffic for every page you visit. But you'll notice a slowdown while surfing and having aMule saturating your connection. But when you down/upload a large file via FTP UBT will recognize this and throttle the upload speed.)