"Landing her first." Of the two most important things I've learned to do in order for a tango to work is (see previous link) timing your lead to coincide with landing your partner first. The second is the active practice of giving and taking space from my partner.
Allow me to explain...
Taking her space
My first tango teacher once told me, "Occupy the space your partner just left." Not knowing anything about tango [at the time], I had no idea what he was talking about. I was simply muddling through my steps.
A year or so later, the light bulb lit inside my head. I finally learned how to lead with my chest by communicating my intentions with my upper body, moving my partner. When I move my partner from her current position to the next, my body occupies the place where she had been. When her upper body moves, my upper body takes it's place, moving her legs. When here leg moves away, my leg takes it's place. All this while leading the pace, distance, and tempo from move to move.
Giving her space
The other side of the coin from taking her space, is GIVING her space. Where taking her space works well for tango walks, giving her space works even better for moves requiring directional changes (e.g. turns, sacadas, paradas, etc).
I found the reasoning behind the milonguero posture that (done right) makes my partner and I look like the letter "A." Back straight, chests projected forward, allows the legs space to move.
The significance of this is that, should I decide to lead ochos, giros, walk backwards, walk on three tracks, etc, it gives my partner a place of her to place her leg to land, thus allowing her space and time to shift and complete her weight change, move without knocking knees.
My lesson learned from this epiphany is that this principle follows common sense and is a fundamental skill, requiring the right posture, leading technique and timing.
Done right, the appropriate giving and taking of her space facilitates that magical tango feeling of having "One body, four legs."