The subject of the Calculate skill may take his action back and choose another action (if the Calculate skill succeeds).
So for example if a Protocol Robot and a Mutant is out on a walk and the walk right into a battle. If the Mutant during his turn decides to shoot an enemy with his scrap rifle, the Protocol Robot can declare that he will use Calculate on that action. The Protocol Robot rolls for his Calculate first and he gets one NUKE. The Mutant now rolls for Shoot and he fails. The Mutant can, thanks to the Protocol Robot, select another action (this must be done before the Mutant pushes his skill roll).
So the Calculate skill is a failsafe, if the subject fails his skill roll. If he succeeds with his skill roll then, technically, the Protocol Robot has wasted his action (well, if the Protocol Robot got a stunt the subject got +1 (per stunt) for the action he wanted to do and also the action he chooses to do if he selects another action).
(This is according to the Swedish rulebook. The English version may differ).
"War, war never changes" - Lone Wanderer