A Skill Flip occurs during the attack resolution: both players flip a card off the top of their respective decks and their skill is added to the current attack/defence total. If the Skill Types on the flipped card match the Skill Types of any of their cards in play, they add +2 to the attacking/defending total for the round.
If a Goalkeeper is revealed as a Skill Flip while defending an attack on your goal, the Shotblocking ability is activated and you add the amount specified to that of your defending card. If a Goalkeeper is revealed as a Skill Flip at any other time, it is worth 0.