Ive Installed a few of them in past, although the stuff we used cost a lot more than that one, they work on Same Frequency Repeater principle and can be a bit hit and miss if not Installed properly. The trick is to get as much isolation between the 2 antennas as possible, normally done by getting the Source Antenna as High as possible on the roof and as far away as possible from the inside Home Antenna.
The inside Home antenna needs to be away from Windows etc best archived by mounting in the ceiling of the target Room or in the center of the Ground Floor, dont expect it to cover the whole House, it will only cover a few meters from the inside antenna, you can get splitters and use more than one inside aerial to different rooms etc, if its a big house though.
Also make sure you using a really good low loss coax as well that is also key, by they way these are not Legal to use as your Re Transmitting the Networks Radio Signals and you dont have a Licence for that, you can legally buy them and Install them its the switching them on thats the illegal bit, the Networks know about them as do Ofcom as far as I know no one as been prosecuted yet.