Well, for me, as I was taught, it is the solar plexus, maybe the upper solar plexus, I dont' know. It depends on what feels most comfortable. The business of standing a foot apart is well, do we all want to pretend we're dancing like Gavito? I tried that once with a friend and it did create plenty of space down below but it was murderous to maintain and certainly not relaxed or suitable for dancing several tandas.
Yes I know that is taught but my reference isn't teaching and of course you are right
about the disadvantages of excessive distance.
A few inches apart with the feet is about right. Could be three, could be four.
Dance isn't static so even this distance can disappear and I work from the basis
that it's the man's role to know exactly where he is taking his partner and take
feet avoidance measures. Too often in modern tango it seems to be left to
to woman - think of Noelia Hurtado and Carlitos where she provides almost
excessive foot space with an exaggerated bodily distortion.
But if you bend forward at the hips (you, the leader), you then have to straighten up your chest. It's a two-part hinge. The other part of the story is critical--the abrazo has to feel comfortable and natural and sustainable throughout the evening.
Here we disagree. The bend is quite a slight one (if both partners hinge then
it becomes almost invisible but it can definitely be felt) and it's a single hinge
at the point in the body designed to hinge - the hip joints. If you have adequate
core strength the torso remains straight and should remain straight. If a lumbar
curve is used to "straighten up" the chest (especially if you are aiming for solar
plexus contact) the body is stressed in a different and possibly harmful way.
The most relevant result of this way of connection is that both torsos are in
natural alignment with no stressful spinal curvatures. Both partners can twist
(and together) as much as standing upright yet retain the upper chest connection.
And to address your final critical comment:
Both partners too retain their own balance without dependence on each other -
the ladder metaphor is redundant - and dancing for hours is easily achievable.
This isn't a method I've heard of being taught - you have to discover for yourself.
One final point - fitness is key to this, your core needs naturally to be present
and engaged, Possibly this is the reason this came naturally to untaught
Argentines in Golden Age and does not to today's often sedentary bodies.
Plus teaching in classes is reductionist, addressing the widest audience
with the most universally achievable.