Activation base is in Stamina and values vary with fatigue%.
Benefits from Assassin and Shadow talents based on backstabs apply for backstabs, but not critical hits.
This talent's effect applies to both ranged and melee. It must be activated with melee weapon equipped, however. Afterwards, it is possible to switch to ranged and still benefit from 15s auto-crit. Sadly, this seems to not always be the case: In the Steam version of the game, Pinpoint strike goes away when you switch from melee to ranged weapons.
This talent works weirdly with Dual Striking and Rapid Shot. All attack rolls which would normally be critical hits are converted into normal hits. All attacks which would normally be normal hits are converted into critical hits. This means if you want to take advantage of dual striking or rapid shot + pinpoint strike, the lower your crit rates, the more forced crits you'll have.