That is not at all the distinction between decisive and withering.

From an in-character perspective, EVERY attack is an attempt to stab someone, barring gambits. The difference between withering and decisive is purely a meta thing (or a dramatic thing, but definitely not something your character knows).

Successful withering attacks draining your target initiative to boost your own is supposed to represent gaining ground, getting a better position and and gaining advantage over your opponent. Soak is how effective your armor is in shrugging off blows or absorbing them...