Lower back is a primary tension point for me . . . actually it's where it screams for me now. With practice, In a year hopefully it will have moved to my quads . . . two years, my calves and shins . . . three years, feeling into the ground . . . at some point before I die, hopefully I'll be able to move with connection.
Something which I focus on is the waist / hips as well. Everyone has their own stepping stones into this training though. For me getting them to free that up early leads to a better use along the learning curve. It is a defining movement for most MAer's. You see where they generate power from and you can feel the tensions in the tight hips or one-side weighted movement crowd that leads to easier throws. Waist drawing a spiral "through" the groin frequently means they hit a wall when they try and throw you and they end up throwing themselves. Its part of that "magic feel" before they go over without much effort. Since the two methods of movement are so diametrically opposed, the more stable one (all conditions equal) wins. Its another reason I hate ukemi and what it does to people.After teaching people how to fall, I dump Ukemi as a training model and teach them how stand up against significant power and change it. Aiki- and aiki to absorb and refuse to fall with using aiki.