I personally would never, ever do it like the guy in that link.
I go down until I can't keep my back completely straight, then I reverse the movement.
You should feel it in the hamstrings, not the lower back. If you have 'bad pain' the next day, modify your technique.
At the end of the day though, it's your body and only you can decide what exercises you think are safe, and how you are going to perform those exercises.