The idea behind this cup is to challenge a friend to fill it a much as (s)he can. You can also ask two friends to successively add water into the cup, and the first one who makes the water spill out looses.
There is a visible hole in the mouth of the snake. So it is obvious that the water should not reach its mouth. But there is another hidden hole inside the cup at the base of the higher loop. The hidden hole does not act immediately because the higher loop actually forms a siphon connected to the outside bottom of the cup. So as soon as the water level goes higher than this loop, this is not a drop that will spill out but the whole content of the cup: got the trick?
By the way, "Pyth" does not stand for Python, but for Pythagoras. See this Wikipedia article for more information on the mechanism.
Warning: do not use hot beverage to play with this cup!