Play starts with a 2v2 attack. The team attacking must attempt beat the defending 2 and score into the main goal. Once this phase is complete, the 2 attacking players must transition onto the opposite side of the grid where they'll become defenders and defend in a 2v2.  

The transitions are continuous. If you don't have 2 GK's to add to your session or 2 main goals, you can add mini-goals to the practice instead.