It actually doesn't extend the length of the video signal. The HDMI spec does not include any max distances, rather it only specifies a minimum signal quality at the end of the cable. Typically it is possible to meet spec in cables up to about 50 feet, but cable manufactures have made longer.
The longest I have ever seen advertised for passive hdmi over cat6 is 100 feet. These typically come with a warning that basically say that if you add any other connections you are likely to burn up the output amp on one of your devices. The reason this warning is there is because at 100 feet the cable no longer is meeting spec. The spec allows for some head room, while these passive extenders try to push everything to the limit, even if they don't meet spec. You also become limited at that range in other ways such as being limited to 1080p with no support for higher resolutions, which is not an issue for most people.
Now there are HDMI over Cat6 converts that might look to be passive, but they will actually pull power off of the hdmi cable (there is a 5v pin with max of 50mA) in order to add an active amp to get longer distances.