One of these will get you safely down if you fall out of the stand and are tethered but does not take up tension while climbing up.  It's a one time use in case of a fall

http://www.robinsonoutdoors.com/tree-spider/livewire.php

No direct experience with them as I'm not a hunter but have heard that hanging in a harness after falling out of a stand can kill a person the same as a fall as the harness cuts off circulation in a relatively short amount of time. So even if you can call for help via cell phone until they arrive at a remote location it could be too late.

https://www.youtube.com/watch?v=BXy1Aeg1jjg