It's not exactly going back in time, but more like moving fluidly through it to different points. In order to SHIFT, you have to be in a deadly amount of danger. While our consciences still live on, the bodies we leave behind end up dying and so do the other versions of ourselves. There have been timelines where we've been shot, poisoned, melted, exploded...[...not great!]

Carlos is the one who figured out SHIFTing first. Once we learned from him, the others started catching on and we were able to jump timelines to get to the one where we were all still alive. Or, at least, according to Carlos. I never made it that far...I was in the middle of the SHIFT when we woke up here.