I'm not an expert. I believe the principle of it is thousands of bots try to access the site at the same time. All of their requests overload the servers which makes them unable to process all the requests. The result is the site is unusable because each site's servers are sized to handle a certain maximum of requests. Malicious people pay organisations to create these bot invasion attacks when they want a site to go down. The Privilege Checker that you see is a way for Voat to make sure that what is trying to access the site is a person and not a bot. Someone should add or correct me if I'm off the mark.