Traditional deadlift is a great exercise, but it is intended for multiple muscle stimulation as a mass movement. Deadlift is not meant as an isolation exercise where you target just one muscle, while relieving work load from others.
There are ways you can partially achieve this, though. The first thing you must understand, is that if you want your deadlifts to work more of any particular muscle, and less of another, than you cannot do them the traditional way, you need to tweak the movement accordingly.
If you want to target more of your lower back during deadlift, than the best thing to do is not to focus on whether your legs are locked or not, as this only stresses the tendons and insertions differently.
In order to target your lower back while performing a deadlift type movement, you need to change the range of motion. In this case, for the lower back, only focus on the top portion of the movement. When lowering the bar, only allow it to drop just below your knee, and then begin to raise it back to the top. This keeps minimum stress and workload on the glutes and hamstrings, while keeping maximum workload on the lumbar.
This was originally made popular by Dorian Yates, and was first seen performed intentionally and consistently in his movie "Blood & Guts".
