Players can watch the World Championship in-game and earn in-game rewards during breaks.
More specifically, after each match there will be a Team Rumble among those watching in Fortnite. There are challenges that, once completed, unlock RLCS-branded items.
Additionally, players can unlock in-game rewards, including a new theme anthem and banner, in Rocket League.
It's fair to say that Epic Games is now cross-promoting in full force but it's currently unclear how the viewership impact of the activation will be identified, if at all.