You're correct, this doesn't have anything to do with split() itself. split() returns a list. List and other sequences support "slicing". Documentation about it can be see at Sequence Operations. It has the start, stop, and stride values separated by colons.
The start value of 1 says to return the element from index 1 of the list (skipping index 0), the lack of a stop value says to continue until the end, and the stride value of 2 says to return every second object.
The start value of 1 says to return the element from index 1 of the list (skipping index 0), the lack of a stop value says to continue until the end, and the stride value of 2 says to return every second object.
>>> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9][1::2] [1, 3, 5, 7, 9]