Planting of the submissive lips to the foot or shoe or boot of a Dominant Women is often seen or assumed to be the joy of the foot fetishist. Well for many this is true, but for the genuine submissive, this is not a personal enjoyment thing, it's the ultimate demonstration of one's insignificance compared to that of the Mistress. A symbolic act of presenting one at the most lowest layer, head showing deference to foot. The genuine sub/slave does not get sexual gratification from this act, it's about a physical mark of complete respect and surrender to the ultimate totum, the Supreme Female.
I would assume any act that makes a submissive feel more submissive would invoke that sexual satisfaction one way or the other regardless if it was their fetish or not.
Kissing my Wife's (KH) feet or ass is a big turn on for me. It's not a long drawn out kiss...Just a few to each foot and both side of her ass...I kiss her ass goodnight and in the morning when we get up. She will wait for the kiss, then get out of bed...Gets me hard in my cage every time...