The power of a voter or block of voters is the number of ways in which it can cast a critical vote that changes the outcome of the election. By leaving a winning coalition, a critical voter changes it to a losing coalition. And by joining a losing coalition, a critical voter changes it to a winning coalition. Needless to say, these ideas apply directly to a wide range of political and corporate situations. A couple of quick examples show how it works.
Company rules say that a majority vote is required to approve any measure.
Clearly, it takes two stockholders to approve any measure. Although one stockholder holds far fewer stocks, they all have the same effective voting power. Again a majority is needed to make a decision. Now any two of A, B, and C command a majority. However, D, who holds only slightly fewer shares than the others, cannot form a winning coalition with any of them.