夅 is the old 'two feet stepping downward' figure behind 降 and is not used independently in modern Chinese. It supplies the sound and 'coming down' sense in 降 (to descend / lower; also 降 'to surrender') and 隆 (grand / rumbling).
memory
Two feet, one above the other, both pointing downhill: a person walking down a slope. That descending motion is the whole idea of 夅.
