Similar to the Framing Square, the marking ink pot is an important piece of equipment for a Japanese carpenter.
A pin is tied to the end of the string, which wrapped around the wheel of the marking ink pot. When the string is pulled from the wheel, the string goes through the ink pot and when the required straight line is positioned by the tack(pin), the string is stretched upwards by your fingers and snapped back to mark the target material with the ink from the string.