There was a bit of a controversy about this, so here's the official restart procedure, going from yellow to green.
1. The race marshall will inform the drivers that at the end of the lap, the field will be going green. He will ask the lead driver to call green at the start finish line.
2. Cars will exit the last corner, maintaining legal speed (60 mph or less).
3. As the lead car crosses the start/finish line, he will clearly announce either "green now," "green green green," or something similar.
4. Other cars may begin accelerating as soon as green is called, provided they were at or below 60 mph at that point. (In other words, don't jump the green.)
