Okay so there are basically two different versions. The one above is supposed to put more emhasis on the spinal erectors and the way I like is supposed to put more emhasis on the glutes and hams. However, to me the best way to do an assistance exercise is the way that most closely resembles the way you use you muscles in the primary exercises.
I'm sure most people here actually do it like the video above but I don't see a lot of value in isolating the spinal erectors as lifters when that is not really their role. Or at least it shouldn't be if you want to have your back for as long as possible. But if the other way works for people, whatever.
http://www.bsu.edu/webapps2/strength.../gpullthru.mpg
The muslces of the posterior region are viewed as working in a "chain", hence "the posterior chain". If you view them as links in a chain then it should make sense that focusing more on one link over another in a way that puts undue stess on that link may not be the best way. But different things have value for different people and I am not one to say that my way is always the best way.
Keep in mind that I'm speaking in generalities. There are going to be people who, because of the way they are built are going to need relatively stronger lower backs or stronger glutes and hams in order to complete heavy squats or deads. But this does not mean to me that choosing an assistance and performing it in a way that puts you a very different biomechanical postion than when you do the lifts is going to automatically lead to bigger numbers.