Why do we have to "watch for the man whose stomach does not move when he laughs?"

Question

-a Cantonese proverb

Answer 1

Because the laughter is a facade.

He could be the Entity that got on that Qantas flight and went down in Manila.

Answer 2

If there are crocodile tears, there's also crocodile laughter or what we Filipinos call "tawa ni Hudas".  Doesn't come from the heart (not a hearty laughter) thus the stomach doesn't move.  

Answer 3

It could probably be because his laughter is one of pretense.  When one gives a hearty laugh, every part of his body rejoices in his happiness and this is manifested in the shaking of his body, specially his stomach.  Therefore, a man whose stomach does not move when he laughs is a hypocrite, a pretender.

Hmm...I'm not actually sure if I explained it right.  Oh well...