When a minority shareholder has enough voting power to influence or determine corporate policy. Working control exists particularly in corporations with widely dispersed share ownership where no shareholder has a majority interest.
Taobiz explains Working Control
Working control exists when a minority shareholder or multiple minority shareholders unite to possess a controlling vote in a corporation. The opportunity for minority shareholders to gain this control is seen especially in corporations where there is no dominant majority shareholder. While there are no official benchmarks for defining working control, 20% ownership is often considered large enough to exhibit this level of influence.