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 or if the enemy is immune to Impact damage or currently completely immune (grey healthbar). 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.
The mod is most useful on melee weapons with high attack speed that have more than 0 base Impact damage, and on stances which involve applying multiple discrete instances of damage on the same keystroke (i.e., get the highest actual attack speed out of a given theoretical attack speed). 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.
The mod is most useful against enemies with low amounts of base armor, and at high enemy levels.
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.
The interaction mechanics of Shattering Impact with other sources of armor reduction are a subject of ongoing research. At max level, the number of hits required to fully deplete an enemy's armor varies between a single hit for enemies with only 5 base armor up to 84 hits for those enemy types with 500 base armor, the current maximum non-boss base armor in the game.
The Valkyr and Volt warframes with proper mods, using their respective melee attack speed increasing abilities Warcry and Speed, and using Swirling Tiger stance on a dual sword weapon can deplete 500 base enemy armor in less than 10 seconds due to the multi-hit of the Winding Claws combo.
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.