The only gap in oncoming traffic occurs when the oncoming traffic has a red light. But the left-turning traffic also has a red at that point. Which is why you

The only gap in oncoming traffic occurs when the oncoming traffic has a red light. But the left-turning traffic also has a red at that point. Which is why you need to move onto the intersection as the light turns green -- you've now passed the traffic light, so once the oncoming traffic gets the red light, you are clear to turn.