Originally Posted by AndreyT

Linear cable housing is virtually compressionless. However, it has relatively low sidewall strength under high load. If you put a high load on the inner cable, it can "slip" between the linear strands and cut/rip through the housing wall.

Spiral-wound cable housing is relatively "mushy" under linear compression, however it protects the housing very well: it can sustain extremely high loads without allowing the cable to cut through the housing.

Obviously, it makes perfect sense to want to use to use compressionless housings for both applications - shifting and braking. Unfortunately, it is not that easy.

Shifter cables work under low loads. For this reason it made perfect sense to use compressionless cable housings for shifter cables from the very beginning.

Brake cables work under very high cable loads. Because of that, initially it was impossible to use linear (compressionless) cable housings with brake cables: the danger of having the cable to rip through the housing under emergency braking situations was too high. For safety reasons, brake cable housings were spiral-wound, which reduced the crispness of the brakes but hugely improved their sidewall strength.

However, with modern materials and technologies, it became possible to make cable housings with linear metal strands (compressionless), which at the same time have sufficient wall strength to withstand any real-life brake cable load. Kevlar is one of the materials that is used to reinforce brake cable housings specifically for that purpose.

And that is exactly what you observe in case of Jagwire brake cables: the housings use Kevlar sleeves, which gives them sufficient strength for braking applications. Because of that, the housing can finally abandon the traditional "mushy" spiral wind and take advantage of compressionless properties of linear metal strands.