rejoice, open one's heart
忻 centers on heartfelt joy and emotional openness: rejoicing and opening one's heart freely.
忻 combines 忄 (heart) with 斤, which likely contributes the sound. The heart component points to an emotional state of joy or openness.
The heart radical 忄 beside 斤 suggests a heart that is light and open, like joy lifting a weight. Picture your heart rejoicing so freely that it feels weightless.
For キン, imagine a joyful king opening his heart to his people: king -> キン, and his open heart makes everyone rejoice.