"Blame it on the English language. When I used "bracing" I was thinking of the way Alex Moore used the word in his book's section on "Rise and Fall--Body" (referring to what the legs do), which, granted, is not the same thing. What I actually meant was the opposite of "spaghetti arms," what is usually called "tone in the arms." I would have been better advised to say something about the connection achieved by proper dance frame, which apparently is what the original poster's instructor was trying to get across."
I would consider the choice of the word "bracing" in the description of a lady's heel turn a bit unfortunate, as most people read into that quite a bit more muscle activity than what is really desired. What is actually needed there is to simply not allow the arriving knee to bend to the degree which it does in a foot-separating turn. The legs should as always remain as soft as possible consistent with achieving this.
But the situation with the arms is extremely different. Unlike the legs which must carry the force of supporting and accelerating the body weight, in the vast majority of situations between highly skilled dancers, the arms need only carry minuscule communication forces.
"Spaghetti arms" is a well known beginner mistake. But it also comes confusingly close to something that in a more advanced dancer becomes a necessary virtue which world-class coaches will demand when a student is ready. The actual focus of hold activity needs to be in the hands, rather than the arms. Top teachers will insist that you demonstrate an ability to hold a partner securely with your hand, while at the same time leaving your arm relaxed and free to be moved. Needless to say this can be a bit confusing and difficult concept when first introduced! But it's key to a higher level of dance capability to understand that the arm muscles are not activated, and the arms are not braced in position. Instead, they achieve their customary position passively, as a result of joining two bodies which are kept in relationship to each other by information - not by force. The reason that you don't see the arms moving around a lot is not that they are braced, but that they are joining two bodies that are in full cooperation. Nothing keeps the arms from deflecting - but there's nothing to cause them to either.
For the new dancer, spaghetti arms a mistake because they are symptomatic of a total lack of the principle of connection (and possibly, of a distracting "express the music in the arms" habit). In a more advanced dancer, relaxed, floating, arguably un-toned arms are a virtue, because they are symptomatic of a connection that is no longer in the domain of force, but instead has developed into an ability to sense and share the information needed to move the bodies in cooperation, without force.