One Degree of Flexibility

The human knee joint has one degree of flexibility, that is to say that it only bends in one direction (straight back from the way that the foot is pointed).  It does not rotate without injury (ask my "ski knee").

This is the reason that I advocate keeping your feet parallel with each other and with the direction of travel (in a line or curve) during the tango walk as if you were walking with skis strapped to your feet) it is also important that the steps be straight back or forward, as if you were stepping on the tracks made by two such skis...or on two small railroad tracks running beside each other (this idea works better than the ski track analogy when you think of the tracks during a turn)(pivot completely to achieve this) (in other words one should never cross their own center when walking even in a turn) , as opposed to stepping back or forward onto one such track as if stepping on  a balance beam.  Consider this... if your foot is turned out and you place weight on the ball of your foot (as followers should do initially while transferring weight to the extended leg during a back step) the axis of your body is now at the outside of the leg rather than the inside because of the turnout.

There is a lot of talk about use of  "Natural Foot Turnout". This technique sends a conflicting signal to the brain because the foot is pointed in a different direction than the line of travel (this applies to followers as well as leaders).  It also sends  improper feedback to the leader as to the direction that your knee will bend and the direction that your body wants to go naturally (without using extra muscles to achieve the walk).  Ones foot should always be pointed in the expected, or the led direction of travel (don't worry followers navigation is the responsibility of the leader and he will correct it with a pivot if needed but that's one more thing for him to think about
...if you catch my drift) (read as... I have enough to think about without having to steer a shopping cart with a wobbly wheel) .

Try this exercise:
Take a back step using "Foot Turnout" (if the feet were side by side they would form a "\/") transfer your weight to the extended foot completely (do not hold a twist in your core or hip) and then bring the feet side by side and completely relax your body...chances are (if you followed the directions for the exercise) you will end up facing in a new direction (the same direction that the weighted foot is pointed).
Now try the same back step keeping the feet parallel and pointed with the direction of travel (||) (you will remain with the direction of travel when the feet are together).

Another exercise:
Stand with the feet completely parallel to each other... place weight completely on one leg and bend the unweighted leg at the knee...It will point straight behind you.
Now stand with feet together heels touching and let the feet form a "\/" (foot turnout),  When you bend the unweighted leg at the knee it will not point straight back, but rather...it will point across your center line (a little extra weight  to counteract against in you struggle to maintain your axis) Can you feel the bend in you spine that this creates?

One more:
Stand with heels together and exaggerate turnout (make the angle maybe 120 degrees or more), now put all of your weight on the ball of one foot (keeping the heels together), transfer all of your weight to the ball of the free leg and notice how far that your center must move to accomplish the transfer.  Now try the same thing with no turnout,  this will require far less movement of your center to achieve the same weight transfer (believe me, this is a good thing).

 Leaders may (and should) use this principle to navigate, by positioning their feet in the intended direction of their next move or perpendicular to the intended direction of the follower (in the case of a side step...they should also preposition their feet if necessary to present the back of the knee in a certain orientation (think ganchos, liner boleos, back boleos, leg wraps, etc...).   I personally use prepositioning to the maximum extent possible and only pivot on my own axis when necessary, or for effect.(planeos, needles, enrrosques,changes of directions...etc...).   As a side note I will use a lead for pivot to the follower, she must continue to pivot as long as the lead to pivot is still being given by the leader (thus making over rotation possible), and complete the pivot before the moving leg passes the weighted leg (because when the moving leg passes the weighted leg the pivot stops...) Try it.

Feel free to express your own point of view with a comment.