greg everett has an article on the difference between pulling for the clean and snatch vs pulling for the deadlift. that is where i got the knee extension vs hip extension stuff.
he says that the lower back always will be the first area to fatigue / the area one needs to be most careful of overtraining. there isn't a great deal of muscle mass on the lower back so it always will be vulnerable. of course one does want to work to strengthen it - but one also needs to be really very careful to let it recover properly between training sessions so that one does not injure it.
apparently the knee is in a bit of a disadvantaged position at the bottom of a pull. so what most people do is let their ass ride up a bit into the air before the bar separates from the ground. what that does is deload the legs - the quads in particular. most people find they can pull more weight with their ass up in the air (more of a stiff or straight legged deadlift) than they can if they really keep their hips down low and initiate the bars separation from the floor by using knee extension / their quads.
in a max effort deadlift... the hips typically will rise a bit early. however, the thought is that you want to train for your legs to get stronger as much as possible so they are capable of contributing more on max efforts. also, using the legs more and the back less allows you to train deadlifts a bit more since you won't fatigue your lower back as much and your legs are quicker to recover.
so... um... the thought is that the lumbar spine overtraining / fatigue isn't because the lower back is weak (from a 'must train it more' pov). um... it is more that the legs are relatively weak so aren't contributing their proper share and the back is the victim of needing to do more work than is good for it.
hope this makes some kind of sense.
really vids will help see what is going on with your deadlift.