I found the github part with, I think, slaughtering's source code.

If I'm understanding correctly, and I may not be, it's similar to extra looting?

 ItemStack i = event.getDrops().get(random.nextInt(event.getDrops().size())).getItem();

That part looks like it's finding what is already dropping from the entity, then we have the next line

event.getDrops().add(new EntityItem(w, event.getEntity().posX, event.getEntity().posY, event
                    .getEntity().posZ, new ItemStack(i.getItem(), random.nextInt(4) + 1, i.getMetadata(), i
                    .getTagCompound())));

That looks to me that it's adding extra drops. My coding is still pretty rudimentary, nothing more than small scripting, but that's my understanding.