Caucusing is the practice where a portion of the membership of a voting body agrees to vote as a block, even though some members of the 'caucus

Caucus

A caucus is a meeting of supporters or members of a political party or movement, especially in the United States and Canada. As the use of the term has been expanded the exact definition has come to vary among political cultures.-Origin of the term:...

' might be inclined to vote the other way.

The motivation for caucusing is that it allows members of the block disproportional influence in the outcome of the final vote. For example, suppose that one third of the members agree to always vote as a block. Once they decide how they vote, it will require only a quarter of the remaining voters to agree for the will of the caucus to prevail. That is, a narrow majority in the caucus will override anything less than three quarters of the remainder. As an extreme case, if only "fifty percent plus one" of the caucusing voters support a motion, and one quarter of the rest (plus one) support the motion, it will succeed, even though almost two thirds of voters are opposed to the motion.

The motivation for agreeing to vote contrary to ones own natural inclination on an issue is the knowledge that one will enjoy the support of the caucus on other issues.