A fantastic exercise yet a difficult one to do well, hence the video.
All too often people get injured doing exercises simply because of poor or bad technique and this is because they've not been able to isolate the area they are trying to target and therefore exposing the body to forces at angles it clearly has trouble dealing with. This is how injuries occur.
OK, enough of that ! The simple truth is that you need to watch the tutorial video, understand what you're about to do, and then do it in perfect form if you want real results otherwise what is the point ? Don't just exercise and go through the motions, instead make your time exercising worthwhile and demand results from yourself.