Can you share a particular source? I've actually had an idea in the past about changing rigidity in soft armor to reduce trauma. It's most likely unfeasible, but it would be interesting if someone already built something similar (as is the case with every reasonably patentable idea I've ever had).
There are many examples used as limb & joint protection for motorcycling and mountain biking. D3O, xmatter, SAS-TEC: brand-names of various non-newtonian shear-thickening substances used to allow flexibility and conforming to body motion but firming up under hard/sudden impacts to spread the forces. Not quite as soft or flexible as a fancy sci-fi material would need to be for a full-body armor, but certainly much more comfortable than the hard plastic materials previously used for protecting extremities.
These are often demonstrated by protecting a hand from a hammer blow, and I know from experience they are very adequate for protecting knees and elbows from trees, hard ground, and moderately pointy rocks. However, it's unclear if the shear-thickening is significant enough to protect against an actual sword strike or knife stab, and very unlikely that they could withstand a modern projectile weapon.
The idea is proven, but would need some further materials breakthrough to approach the level of sci-fi armor.
Not combat-armor, but a lot of motorcycle protective gear is exactly this. D3O is soft/flexible normally, but goes rigid on impact. I have plates of it in my own riding gear. I believe there's also some progressive versions that harden more or less based on the impact force.
