D3O stays soft and flexible with you when you move it in a normal manner, yet when it is introduced to any form of impact the offshoots lock together and become hard within a 1,000th of a second [within one millisecond] to absorb and spread the force, significantly reducing the effect of impact.
This transformation is possible due to the way these unique molecules move with each other. When the material is being moved at low speed it is soft and flexible, because the bond between the molecules is not that strong, so they can slip past each other. Yet when the material is asked to move quickly by a sudden hard force the molecules re-arrange themselves in a very structured way and create proper hydrogen bonds. So in other words, when you impact the molecules they do not have time to move and they quickly lock into position.

https://en.wikipedia.org/wiki/D3o