The primary method of action for activated charcoal is to bind to a substance in the digestive system and prevent its absorption into the body.
Unfortunately there can be a significant delay between ingesting the toxin and activated charcoal. However, substances can pass in both directions. So by lowering the concentration in the intestine it also increases the amount that diffuses back into the intestines after having entered the bloodstream.
Enterohepatic Recirculation is heavily influenced by diet which is why fiber and specifically leafy greens seem to be helpful for liver heath. https://en.wikipedia.org/wiki/Enterohepatic_circulation
This is also one of the reasons activated charcoal is useful after ingesting a wide range of harmful substances.