I say 'physical' because it then becomes about the embrace (abrazo) and the physical connection to best achieve that communication (there's probably also an element of telepathy!).
And weight distribution. Many followers, despite being forced onto tiptoes by high-heeled shoes – and more than a few leaders – carry their weight too far back. You want both partners to carry their weight over their metatarsals (not all the way into the toes, which wrecks the toes!), so that each is pressing
lightly against the other. This state of dynamic tension means that little effort is needed to get big results – it's what aerospace engineers call "controlled instability" (here, "control" means keeping track of your axis, while "instability" means sacrificing a bit of your axis to your partner), where the natural tendency is toward rapid response rather than resisting change.
This is
hard. It won't happen in a beginner class. Advanced dancers sometimes dislike it, too – typically due to having heard some teacher say "never sacrifice your axis" as an imprecise way of saying "never lose control of your axis" (partial sacrifice, and just how much, is
control!). There are entire sub-styles of tango where there's no body-on-body press. There are people who avoid it out of prudishness, and others who can't affect it because their joints are in bad shape (rarely
because of tango, but it still
affects tango).
Hence many discussions about body shape, size, and 'fitting together' - and not leading with your arms.
With good weight carriage and axis control, the arms aren't even implicated, beyond offering guardrails that frame the embrace. I can – after 13 years of practice, 10 of those with my current partner – lead all of my vocabulary but colgadas and partial soltadas (where you retain arm contact and only that) – sans arms. But that's hard, too . . . and strangely, the "simple" walk is among the hardest of things to lead that way, because it's hard to maintain a consistent distance and timing without near-flawless weight carriage and mastery of all the stuff I said earlier about indicating, projecting, resynching, and transferring together.
Walking forward (for the leader, backwards for the follower) is about indicating the leg to start with, the intention (with your chest/upper body) to move that follower's leg, and then actually moving it, whilst the leader steps into the vacated space.
It's essentially a series of linear sacadas: One person moves in response to the other person, who takes their place as they leave it. One exercise that helped my walk immensely was actually walking straight ahead in sacadas, placing my right foot behind my partner's right foot (across my body) and displacing their leg, and then doing the same left on left, in effect walking with crossing steps that expel my partner's legs. After that, it's easier to feel an ordinary parallel-system walk.
(It's also fun to use that exercise as an actual expression of the music, but in small doses.)
The follower also has to play a role there and offer appropriate resistance!
Exactly! That's the follower carrying weight over the metatarsals, creating light body-on-body pressure that gives a tendency toward rapid response rather than resisting change. The problem is that sometimes, followers (especially beginners) interpret "offer resistance" as "resist change." They aren't the same thing. Properly offered resistance actually
enables change and accelerates response.
The worst. I danced with her last night. "Where are your legs? Where are you? What are you even doing?" As I say all the time, "lightness" is rapid change, enabled by resistance as I just wrote. Lightness through the absence of resistance is useless for tango.
They're not a rag doll responding to your 'command'! They can stand and move on their own.
Although it doesn't help that there are leaders who
want their followers to be ragdolls, and who use force and words to make that clear. These guys try to move their partners instead of asking them to move with suggestions delivered through intention. And followers who become habituated to such bad leaders often end up waiting for commands and to be shoved around, and so won't take steps in response to a light shift of intention. This flaw is common among beginner followers who dance with leaders who teach on the dancefloor . . . because most such leaders can't silently communicate, but need to verbalize and force.