Forbidden Dreams
Most guys these days dream of getting a rock hard six-pack for the abdomen and most artists draw the same kind of stuff, so it's interesting to explore the opposites, some character playing with its softness and vulnerability ^-^
Which once again reminds of the Tao Te Ching verse (translation by J.H. McDonald):
"The living are soft and yielding;
the dead are rigid and stiff.
Living plants are flexible and tender;
the dead are brittle and dry.
Those who are stiff and rigid
are the disciples of death.
Those who are soft and yielding
are the disciples of life.
The rigid and stiff will be broken.
The soft and yielding will overcome."
...Ok maybe it wasn't about bellies in particular but medically, though, this practice is totally understandable, since there's a huge amount of very sensitive nerve endings on the surface of the belly button, which gives all kinds of pleasant sensations while massaging the naval area.