When the participant has finished marking their statements (or they run out of string), they can set it the stone on the ground, or allow it to dangle from their last statement.
Some rolls of yarn may be more used up than others- some may have the stone exposed, some may be still covered, and a few may be left suspended- used up before they reach the ground.
Just as we all have differing amounts of life without initially knowing how much, the yarn is different lengths.