I honestly don’t remember where I read it. It was either some insurrectionary anarchist thing or an ELF/ALF thing. The gist of it is:

You have two organizations that legally never meet. One stages protests, does community outreach, fundraising, and all of your standard environmentalist org stuff. The other does illegal direct action activities, divided into independent cells and/or affinity groups. The above-ground network serves to raise funds and recruit for the underground network, but the key is that the leadership of the above ground group can never be connected to the actions of the underground group. This gives a lot more wiggle room to both groups than if they were to go it alone.