Master and Servant: When Wong is adjacent to a character named Dr. Strange, modify Wong's attack value by +1 and Dr. Strange's defense value by +1 if not already modified by this effect.

Backup Spellcaster: If a friendly character with the Mystical keyword is within 4 squares and line of fire and can use Perplex or Probability Control you may give Wong a power action to use that power until your next turn.

How exactly does Backup Spellcaster work? If I want to use Probability Control when my opponent attacks, do I give Wong a power action during the attack?
The power action is only to activate Backup Spellcaster, not to activate Perplex or Probability Control. So, if you wish to use Probability Control, give Wong a power action during your turn. Now that you've done that, until your next turn Wong can use Probability Control as described on the Powers and Abilities Card (once during your turn and once during your opponent's turn).