Chris Bacon wrote:

Aha! "Modern indexed gears", hmm.... that's why. One would have to be
a bit of a dunce to use these cables for brakes! Surely Mr. Blackburn
does not mean that the outer of spiral-wound Bowden cable is really
compressible? That would sort of render brake cable pretty useless!

It is compressible only in that it isn't compressionless! Since the
longitudinally wired outer is designed as a compressionless alternative
to the more traditional cable housing, it stands that the original outer
isn't compressionless and so must, to some extent, be compressible. The
degree of compressibility is, of course, slight, as Andy said:

Tension cable outers use a single wire spiral. This is more flexible,
and it also changes in length (slightly) if you bend it, causing the
apparent fake movement of the inner cable.

This effect is used for secondary braking systems (cross levers on the
tops of drops) that push outer rather than pull cable.

Colin