Shield will completely block 100% of automated spam-bot comments.
Unlike Akismet, it does not send your site comments offsite for processing with a 3rd party.
There is just no need for that - we respect our own, and your own, privacy too much to tolerate that sort of nonsense.
It's a simple yet highly effective system for detecting an automatic Bot comment.
Each time a visitor loads a page with a comments area, the visitor uses a hidden comment token to post a new comment.
SPAM bots don't use these special tokens so we can detect this and reject their comments as SPAM.
The timeouts on the settings pages (cooldown and expiration) are for the hidden comments tokens and are necessary for the system to work - you can normally leave these at their default values.
The vast majority of comment spam is automated - bots (see above).
But you get random human spam comments too which can easily bypass SPAM filters. This is natural, because they're real humans. And humans know how to click checkboxes.
This is what the human SPAM detection is for. It uses a dictionary of common human spam terms and if it matches them, it'll flag it as spam and then you can review it. It helps with the decision process.
Of course, it's not perfect. But then nothing out there is 100% guaranteed for human spammers. That's why spam is a problem.