Shattering Impact takes effect on every melee hit from weapons with any Impact damage. It has no effect if the melee weapon's Impact damage is zero. Note that it is not necessary to proc the Impact status effect to trigger the armor reduction.
The amount of armor reduction per hit is a flat, constant value, independent of the amount of Impact damage inflicted. Since the reduction is applied to the target's base armor, for a given enemy type it will always take the same number of hits to completely drain the enemy's armor, regardless of its level.
Compared to Corrosive statuses, the armor reduction of Shattering Impact scales more steeply into higher enemy level regimes, as the number of Corrosive statuses required to fully deplete a target's armor scales logarithmically with its level, while the number of Shattering Impact hits does not increase at all. However, it should be noted that the armor decrease from Shattering Impact in its progression is linear while that from Corrosive statuses are exponential, so even though Shattering Impact may nullify armor faster, Corrosive statuses' armor reduction might be quicker initially.
As of update 18.1.4 Shattering Impact doesn't reduce enemy armor when used with the charge attacks of thrown weapons. The charge attack from a Blade and Whip type weapon will however reduce enemy armor.
The mod is most useful against enemies with low amounts of base armor, and at high enemy levels.
Valkyr and Volt, with proper mods, can deplete even high base armor is a short time using Warcry and Speed, and a stance with multi-hit combos.
The mod is most useful on melee weapons with high attack speed, and on stances which involve applying multiple discrete instances of damage on the same keystroke. Due to its demand for high attack speed, this mod has good synergy with Fury and Berserker on weapons with decent amounts of base critical hit chances, and by extension with True Steel or Blood Rush, as they both facilitate the build-up and maintenance of the Berserker bonus.
This mod does not have a description in the Polish and Spanish Warframe Clients.
Very often the armor reduction from Shattering Impact appear to only be able to be applied by the host as the health bar remains yellow for the clients. However, the actual effects from the armor stripping are applicable for the whole squad.