Chatushpada is the second of the four fixed karanas and holds a single position in the lunation, position 59, which is the first half of tithi 30. Tithi 30 is Amavasya, the new moon day, so Chatushpada is the half-tithi that runs up towards the conjunction of the Sun and Moon.
The position is not incidental to how the karana is used. Amavasya carries the observances for the ancestors in much of the tradition, and the two karanas that divide it, Chatushpada and Naga, are the only two that ever occur on it. Between them they account for the whole of the darkest day of the month, and they occur once each per lunation and never anywhere else.
The four fixed karanas exist because sixty is not a multiple of seven. Removing four from the sequence leaves fifty six positions for the seven movable karanas, which is eight complete rotations, so the cycle closes cleanly at the new moon instead of being cut in the middle. Three of the four fixed karanas were placed at the end of the waning fortnight and one at the beginning of the waxing, which gives an unbroken block of four across the conjunction: Shakuni at 58, Chatushpada at 59, Naga at 60 and Kimstughna at 1.
The duration is the general one. A karana averages 11 hours and 49 minutes, one sixtieth of a synodic month, ranging from about ten hours to about thirteen and a half as the Moon's speed changes.