To be frank it was never explained. It sort of just happened after Soul Society Arc ended, granted there were instances during the first arc where Ichigo "stood" on the air but that's about it.
Orihime was too much for OP 4 i never noticed there were different versions Does anyone remember the second version of OP 7 with the blue flames that was pretty much an ad for the Diamond Dust movie or was that another fever dream?