The subspace weapon is actually quite simple in effect - it resonates a section of the ship's hull instead of its entirety, thus ripping that section into subspace and reducing overall hull integrity due to massive stress. The effect depends on the power of the weapon - a single Sathanas would be able to quickly annihilate a destroyer with it, while the Gigas can cause exponentially more damage.
The Icanus beam is far more effective against ships due to its faster powerup and recharge rate, and vaporises nearly all naturally occuring and artificial materials in an instant - hence why you see a destroyer hit with it go from 100% to 0% in a single frame.
When used on planets, it bores a hole so deep, and causes so much damage to the planet's structure, that the resulting tectonic activity would wipe out the planet's surface in a couple of shots... if not cause it to break apart entirely.
