It is not one click as described. This opens up a webpage where you then probably click to download. When I seen that I didn't even click anything there to test if it worked. (just to be on the safe side of things.)
Edit: To the developer. I checked my browser history and I believe this extension used to go to vidscrab.com and then on that website there was probably a download link. Back then I moved on to a different downloader extension but I did retry this one and it works as a 1 click downloader now.
Thanks a lot to developers for such a useful tool! I really enjoy the synergy of functionality, usability and continuous perfect work. I'd like to mention that this add-on saves the most valuable thing in our life – it saves the time, ‘cause just in one click any video from YouTube is on your computer. The only problem for me as for a fan of Chrome is that the Google’s browser hasn’t got such a necessary extension in so high-end implementation!