Fused Together is a major twist that featured in Survivor: Kazakhstan. The twist swapped out the basic starting concept of two tribes in favor of having the castaways battle in individual competing pairs. The twist returned for Survivor: Indonesia, and for one day in Survivor: Entitled vs. Earned.
Under the Fused Together format, instead of competing in two tribes, the castaways are now in pairs of two. Immunity from Tribal Council is granted only to the top three/four/five pairs in each challenge. The losing pairs must attend Tribal Council and vote off a single castaway. The person who is robbed of their pair will then be immune for the next round until another pair falls, and the two surviving members become a new pair by default. It's a dramatic change to the standard season format, and continues to play out until the host either merges the remaining castaways, or diffuses them onto two new 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.