The instructions:

Stamp it in a save and delete the 40th particle in the save (use debug mode and find particle with the number 39, or find where the soap is all pointing towards). It will target the next 40th particle. The 40th particle must be liquid or powder, and must stay on the play area for it to attack. 

If you follow the instructions properly, it will attack any particle with pressure!

If you're asking, I found it on accident when I was "injecting" different colored BIZR into a bleeding glass dummy. It suddenly started destroying the dummy for no purpose when I turned all the BIZR in the save to SOAP through console and removed the targeted particle.

Explanation: It isn't really a glitch or a bug. The tmp of the SOAP will target the particle with the same particle number tag in Debug Mode. When being targeted, the particle is being affected by a "hidden velocity" so the particle sticks along with the SOAP bond. However in this case, so many SOAP bonds are connecting to a single particle that the velocity makes it resonate in extreme amounts and create a pressure point. 

Because of this feature, it is also possible to change the target through console and Debug Mode.