My Tab S 8.4 is still unrooted.
I tried going into download mode to just check the KNOX status.
Unfortunately you cant turn it off or pull the battery out!
Doesnt work when pressing Power + Vol UP + Menu or Power + Vol Down + Menu or just holding the Power.
Here's what i did. In Download Mode, Press and hold Power + Vol UP + Vol DOWN
It should restart the device successfuly.
I hope it helps.