Torture

I'm also not clear about the legal basis for 'authorising' it. Does it really just depend on the whim of the President? I'd be grateful if any legal eagles could explain.

In nation like the US where the figure President has the authority of pardon available to them, the law has a greater degree of flexibility. Therefore where an action is not prescribed or specifically prohibited, you could seek authorisation for that action in the interests of a defined greater good (often national security). In the UK, which is much more of a democratic state, it would take a greater degree of debate and consensus to approve such activity.

All this authority then provides for is safety from pursuit of prosecution through the nations legal system. The whim is effectively that of the President, but not to decriminalise an action but to assume culpability for it. In this instance it would be the situation that the torturing individuals justification for doing so has been accepted and approved. Therefore, any prosecution would have to go from the top downwards. If you can't take out the ex-president, you can't take out anyone below them.