"Ugh…what happened?" Grehha felt groggy, unable to feel his body. Even though he had cranked his Secondary Nature of Internal Inertial Gravity to the limit, Resha's attack was still able to send his highly dense body flying away for hundreds of metres.
"It isn't possible to send my body flying when it was that heavy…no, that wasn't it." As he thought further, Grehha realised what was up. When External Inertial Gravity hit him, the effects of his Internal Inertial Gravity were suppressed for an instant.
In that instant, his body's density reverted to its original value, allowing the effects of External Inertial Gravity to send him flying. An instant later, the full effect of Internal Inertial Gravity recovered his body's density to its maximum value. But by then, the momentum had been transferred to his body, sending him flying.