Maybe I'm still puzzled in general when a critical result can be spend if it is not used as a standard crit: the first damage card is dealt faceup.

So with the ISD we will get this Ion Cannons:

" Blue [crit]:

You may exhaust this card to force the defender to choose and discard 1 of his defense tokens."

Does this effect takes place before or after the defender had the chance to spend defense tokens?

Well after: 5. Resolve Damage: The attacker can resolve one of its critical effects. (p2 ARR)

Right. As easy as that.

Thanks.

Same thing goes for the Overload pulse

Blue crit: Exhaust all of the defender's defense tokens

Crit effect is after the defender uses tokens, so he has a chance to use them before you turn them.

Watch this handy video on Critical Effects!