It's all about the core.

Total Immersion talks a lot about keeping a still, neutral head and keeping the core engaged; two very simple sounding things to do that are, in practice, surprisingly difficult.

For our "ship" to be stable, the bow needs to be travelling straight and not wobbling about and the hull needs to be rigid.
I have found both concepts difficult over the years and although I think my head position is generally not too bad, I definitely have struggled with holding my hips and specially the "hip nudge forward" focal point.
But just recently, on observing my SPL up to 48/49 in 50m pool I have managed to start "holding" my hips better and immediately have felt much easier through the water although it has required some concentration.
Visualising a rail coming through the torso (middle of nipple width) all the way through my hips and down my legs on either side has been very helpful.  I think in the past I did not spend much time thinking about what was going on beneath the chest even through I have a competent two beat kick.
I definitely think this has helped me improve a step further and entering the Coniston swim in September has really helped focus my mind back on good efficient technique throughout the whole body.
Goals are good.