The start argument gives the function the starting point. It's what is being added to. So sum([1,2,3]) returns 6 and sum([1,2,3],5) returns 11. In your case, because you're passing in an 2-d list and an empty list, the function is going to add everything in the first argument to the second argument. Essentially, you're doing this:
[]+[1,2]+[3,4]+[5,6]
It's a bit of a quirk of python's operator overloading.
