The navel caressing is just misdirection. Look where Sakuya's other hand is.
On the bed? Or you're saying that Sakuya is pretty good with her thumb and her forearm.
Updated
Reimu's navel, it's so pretty......I'll knock you down, you know.You may say that with your mouth, but isn't your stomach enjoying it?Rub...SakuRei Simple Drawing
Theme : "Navel""A pic of SakuRei quarreling while Reimu bares her navel."Idi... don't say such an embarrassing thing...