This is an interesting feature in terms of temporarily reducing the overall visual real estate, but I think it would be much more useful if you could browse through the collapsed cards in that state, and then expand the ones you want to work with, either by hitting ‘z’ again, or some other key.
In fact, my preference would be to be able to collapse and expand cards either as a a branch of the tree, or individually, as needed, with the ability to expand them in the same way. Just a thought. I don’t know how much work is involved here . . .