Tener la sartén por el mango

Literally: To hold the frying pan by the handle

Example: “Con la única copia del documento, ella tiene la sartén por el mango.” — With the only copy of the document, she calls the shots.

English equivalent: To call the shots

Origin: Whoever holds the frying pan (sartén) by the handle controls it without getting burned, and decides what happens to whatever is in it. Having the pan by the handle is being in charge. The RAE dictionary lists tener alguien la sartén por el mango, and Spanish also says tomar la sartén por el mango.