How to make a dessert with dynamically opening flower petals? Anyone has any idea how to make petals open when sauce is poured onto the plate like pictured in this video? 

I realize it requires a lot of artistic skills to do it on this level, but what is the general mechanism behind the chocolate petals opening?
Gravity is making the petals open. If you look at the shape of the petals their center of gravity is towards the outside of the bowl, but they are being held in place by something underneath. I suspect that it's simply one piece of chocolate melted onto another. When the custard (warm or hot) is poured on it melts the bonds holding the petals in place allowing them to fall.