Sideways = Another Way by Michael S

I believe the Sideways construct was something created by Hurley (and Ben) as "another way," as referenced in the discussion between Ben and Hurley at the mouth of the cave. Hurley, with no idea how to run the island or what it even means, is unsure of his next step and clearly does not like the rules that were put in place by Jacob. Ben tells him that Hurley need not run the show the way Jacob did and that Hurley should use his time in control of the island to do what he does best: taking care of other people. I think Hurley set out from that point to make rules that allowed for the Sideways construct to take hold for all of those that he lost on the island.

Hurley always had the most trouble letting go of the people who died on the island (no doubt caused in part by the sudden murder of Libby). His sobbing reaction to the death of Jin and Sun, his adamant refusal to let Jack die when it's clear Jack intends to go down into the cave, his frequent visits to Libby's grave, all point to a man who does not cope well with the loss of loved ones in the present.

The ability of the Island Protector to craft the rules of the game, I think, led Hurley to create new rules that allowed all of those from the Island to be together in the next world, flush with the memories of their time together and their love for each other.