The standard propositional truth tree diagrams are not a problem. Where I am having trouble is opening worlds in the tree diagram and closing branches with these opened worlds. I don't see why I am justified in closing a whole branch if some worlds in that branch do not have a contradiction in them.
At about 14:00 in the video the presenter states, "If we derive a contradiction in either of these worlds the whole branch closes."
Shouldn't we close all worlds prior to closing the whole branch?
Here is a screenshot of the tree:
I don't see why stacking the worlds justifies the step. Creating world "w2" was not permitted without the stacking.