Empty Pockets,
Taken from the website:
The route has been carefully planned to optimize on climate and light conditions and to remain within the rules set by the FAI (Federation Aeronautic International) which state the aircraft must remain at a latitude of less than 66° 33’ (outside the north and south frigid zones) and cover a minimum distance of 19,850.83nm (equal in length to the tropic of cancer).
The zig zagging is to make up the miles I believe.
Hope that answers your question fully.