Physics Impact damage is one of four damage types that do not apply armor mitigation in damage calculations (others are Serrated Blade, Poison, and Armor Piercing). 

There are currently three enemy types in the game that retain use of armor calculations when struck by Physics Impact damage: Grineer Seekers, Commanders, and Rollers.

Enemies killed by projectiles from these weapons have a chance to be launched backwards upon death and pinned to the wall; enemies struck by the moving corpse both take the base damage of the killing shot and are staggered.

Innate Physics Impact Weapons