GS movement order

By Scottgun, in Death Angel

What is the order of movement for Genestealers in the event phase? Is it like attack order in which GS attack starting with swarm at the topmost position going down and left before right when they are on the same level? Or is it simultaneous? The example on pg. 23 could be read to suggest either.

Thanks.

Each row. First top left, then top right. Second row left, then second row right. Etc.