equal, important point
鈞 centers on the idea of equal weight or balance, often in a metaphorical sense of a crucial, balanced point. The meaning 'important point' extends from this idea of a pivotal, evenly weighted moment.
鈞 combines 金 (metal) with 匀, which likely contributes the sound and the idea of equal division. Historically, 鈞 referred to a unit of weight, hence the association with balance and equality.
Metal 金 weights are used to balance things equally. The right side 匀 looks like a balanced scoop, reinforcing the idea of equal measure and a crucial point of balance.
For キン, imagine a keen balance scale made of metal: keen -> キン, and the metal scale finds the equal, important point.