I've noticed and used the new (at least for me) button Leave Open, but I can't find any explanation as to the implication of clicking it.
I've found a highly voted status-declined question:
...How about, as soon as there's one vote to close, there should be an option to counter the close vote, something like "vote not to close" or whatever. This would then bring the "Vote to close" number down by one.
Is this that is happening? Or something else?
By reading the FAQ, there's no mention to this new action... so I'm assuming that an update is coming to the last paragraph of Why are some questions closed?:
Users with 3000 reputation can cast up to 50 close votes per day. When a question reaches 5 close votes, it is marked as closed, and will no longer accept answers. Closed questions may be opened by casting reopen votes in the same manner. However, you may only vote to close or reopen a question once.