I feel like I am missing something because this seems so mind warpingly broken. So if I am reading this right I:
>react when my opponent shoots anything in my army that could have shot an infantry unit within 12" of the targeting unit
>my opponent takes a cool test for the shooting unit
>if they fail, they shoot the reacting infantry
>if they pass they snapshot the original unit
So I am envisioning the following scenario. I have a few solar aux infantry units in front of an imperial knight. You shoot my knight with whatever anti tank unit you have. I react and either you snapshot the knight or blast a poor las rifle man. If you bring ultima pattern and a command and control squad you could do this up to 5 times if you have enough different infantry units to allocate to. Even if this gets changed so the unit being shot at has to also be infantry this will still lead to untouchable hss or whatnot.