Rubbing my tummy and patting my head

What does "rubbing my tummy and patting my head" mean, and why does it mean what it means?
The example I saw was: "it is like rubbing my tummy and patting my head" referring to the person's not having been able to get the hang of it (I think).

Ever tried rubbing you tummy with one hand and patting your head with the other? It demands a deal of co-ordination and indicates that a task is difficult to execute.

It's kind of a trick you play on a little child. Ask him if he can rub his tummy and pat his head.