How do I backstab?
To kill them in one hit you need to be behind them and be in range, hitting too early will uncloak you and the bot will turn at once.
You can see that you are in range by your crosshair chaning looks as on the picture below.
As far as I can tell you do not need to be cloaked, but I have a hard time telling if the cloak have faded before i hit when I try to do it vs the bots in the training mission and it is definitely easier if you are cloaked.