He will then proceed to "demonstrate" his knowledge by blatantly reading a Wikipedia article or website about the topic. The point of the gag is that he purposefully makes it extremely obvious that he's trying to get away with it.
Since the guests tend to be new to the show, the gag has an essentially infinite shelf life; Joe will pull it out quite frequently, amusing and annoying the guests with obvious typing and clicking while claiming "I'm an expert in that." When called out on the fact that he's reading an article, he'll continue to deny it, saying "I'm not reading an article, I'm an expert on that. This is based on memory."
This gag was essentially retired in late 2019 as the hosts switched to higher quality microphones with less noise capturing, set up on booms and shock guards; the typing and clicking that made the gag amusing would, unfortunately, become impossible to hear.
The gag originated on an episode with Ohan, where he claimed to be an expert on the video game Bloodborne. When tested on this claim by Lars and Ohan (the latter of which stating, "I'm gonna gatekeep you right now"), he can be heard furiously Google searching for the answers.