Recall that the * operator unpacks a tuple into a positional argument list to a function. So if z = zip(a,b) is the (iterator) sequence: (a0,b0), (a1, b1), (a2, b2), .... Unpacking this sequence in the call zip(*z) is equivalent to calling zip with these tuples as arguments:

zip((a0, b0), (a1, b1), (a2, b2), ...)}

zip takes the first and second items from each tuple in turn, reproducing the original sequences:

(a0, a1, a2, ...), (b0, b1, b2, ...)