If you utterly fail a jump and are about to fall to certain death, Elika will project an astral hand that will catch the Prince and place him on safe ground (aka, the last checkpoint). In combat, she will also intercede to spare the life of the Prince. Basically, this is like having the unlimited rewind functionality of the Sands of Time. Ubisoft defends the lifeline, saying that it's really no different than dying and re-spawning at a checkpoint. The only difference is that instead of a "you died" screen, you get an admonition from Elika and a reset to a checkpoint. According to Ubisoft, it helps the gameflow.