Originally posted by KewpieI can't find this option.
It's a way of starting a game that already has some moves made. For example, you've just finished a game where you made a significant choice of move at Move 12. You can play over the game again with the same opponent, starting after Move 11, using the "start from this position" board. Now you can make a different Move 12 and play out the game again and both see the consequences. A good learning tool. Try it out and you'll see how it works.