Taken from the Help file..
Automatic
Counterstrike is a measure of how alert a barbarian always is to any sort
of physical attack. When an opponent strikes the first blow, a barbarian is
nearly always able to turn most of the damage back onto the attacker, doing a
great deal of damage in some cases.
[Skills help] alertness
Automatic
Through alertness, a barbarian learns to be in-tune with his surroundings
at all times, such that he can sense when someone is passing near his
presence, even if they never make a sound. Although dulled somewhat, a
skilled barbarian can even sense people approach while he is sleeping.
This skill is the first step along a barbarian's path of supernatural
awareness that eventually leads to the ability to counterstrike.
Notice the highlighted portions. Barbs, even sleeping, can sense people coming and expect an attack, even if sleeping. As someone said above, barbs don't sleep, they wait, and that is too true.
However, if it was possible to code, I could understand someone having to master or get Alertness to superb before learning counterstrike AS WELL as making the Alertness check to know the person is even in the room with them to counterstrike an attack. As it is though, it's not that bad. Dual backstab can decimate just bout any class other than a barbarian, so just find out who's what. Make sure you don't stab the person again.
