i have little confused.

Counter strike is triggered when you declare the unit as a defender. It doesnt target.

But it does need an active attacking unit in order to even initiate, just like you must have an active attacking unit in order to declare a defender at all.

Rule book page 16, paragraph 2: "Whenever a unit with the Counterstrike keyword is declared as a defender, it immediately deals uncancellable damage equal to this numeric value. This damage must be dealt to a single attacking nit of the defending player' choice; a unit with counterstrike cannot split the counterstrike damage amongst more than one unit."

this means if you are defending and your unit has counterstrike it deals the damage right away and is done to 1 unit of your choice. it isn't pooled and can not be split.

for example: Reiksguard swordsmen is a 2 p, 2 hp, counterstrike 4. If it is attacked by two crooked teef goblins (1p 1hp) and the RS defends then the defender, before combat damage is dealt, chooses one of the goblins and all counterstrike damage is applied to that goblin. the result of this instance is that one goblin would go to the discard pile before damage is dealt, the second goblin takes 2 damage and goes to the discard pile and the RS receives 1 damage and has 1hp left.

The counterstrike could not be split. I hope that helps.