The Queen is inward maturity; the King is outward maturity: not only understanding the element, but governing with it, deciding with it, answering for it in public. All four Kings sit enthroned facing outward — the emblem in their hand isn't for contemplation, it's for use.
The mechanism is responsibility. King and Knight both act; the difference is that the King's action has to answer for its consequences, so he's slower, he weighs, and sometimes he chooses not to move. That's also his limit: the King of Wands' fire is framed by a throne, the King of Cups' water is held in by protocol — the price of maturity is no longer being pure.
Where you'll recognise it: you're the one who has to decide, people wait for your position, when something goes wrong they come to you. Running well, a King is steady power: opinions of his own, able to carry weight, reassuring to those around him. Stuck, his problem is rigidity — too invested in control, unable to hear, turning responsibility into domination. A King should remember he was once a Page: whoever governs an element needs to keep their curiosity about it.
