Battle Royale is a major twist that featured in Survivor: Battle Royale. The twist swapped out the basic starting concept of two tribes in favor of having the castaways battle in individual competing pairs.
Under the Battle Royale format, instead of competing in two tribes, the castaways are put in random pairs of two before every challenge. Immunity from Tribal Council is granted only to the top four pairs in each challenge. The losing pairs must attend Tribal Council and vote off a single castaway. On rounds where there is an odd number of castaways remaining, one castaway had to compete solo, and would either win Immunity for themselves or lose and send themselves to Tribal Council. It's a dramatic change to the standard season format, and continues to play out until the host divides them into standard tribes.
The crucial point of this twist is that castaways are unable to vote unless they attend Tribal Council. That means if one wants to have a direct say in who goes home, they have to risk the safety of themselves and of their partner. Castaways can of course guarantee safety for their pair by winning immunity, but they leave the vote in the hands of the losing pairs only and may not be able to influence the vote as much.