When a body encounters a shock wave, as from an explosive devise, that impulse can ripple through the chest and up to the brain. The explosion creates an increase in fluid pressure that travels up into the axons, particularly those in the cerebellum/brainstem and the optic tract. Fortunately, wearing body armor can ameliorate these effects, as shown by blast tests on mice.
When the researchers compared mice that were fitted with body armor (which conjures an amusing visual) to those who were unprotected, the differences were stark. After being subjected to a shock wave (at a force designed to induce a mild traumatic brain injury), the unprotected mice took twice as long to socialize with new mice and were clumsier. In contrast, the shielded mice performed just like mice that had not been exposed to the blast. Remember, these mice were not wearing head protection, but only body armor. In fact, adding a helmet offered no additional protection.
The authors caution that helmets are still critical to protect soldiers from direct impact to the head. This study makes it clear that in order to protect the head, you have to protect the torso as well.