It took me a bit of time to find out how it actually works, as the 'How it works' button doesn't actually tell me how it works. I had to click onto the FAQ in order to find out that it integrates with Gmail.
Unfortunately I will never be giving a third-party access to my inbox like that. I was hoping that this service would be something where you can forward a spam email and they visit the unsubscribe link for you, or you simply paste in an unsubscribe link and it handles it all.
Perhaps these suggestions could be added as extra features for those of us who want to unsubscribe without giving away access to our inboxes?
Currently my personal process for unsubscribing is to either put the unsubscribe link into urlscan.io, and if that doesn't work (e.g. if it requires a complex form input), then I forward it to a malware sandbox machine and do it there.