Mikvaot, Chapter Four, Mishnah Two
Our mishnah continues to deal with vessels placed under a water-spout and whether the water that flows from the roof, through the spout, into the vessel and then into the mikveh counts as drawn water to disqualify the mikveh.
1) One who put a board under a water-spout: if it had a rim to it, it disqualifies the mikveh; otherwise it does not disqualify the mikveh.
2) If he made it stand upright to be rinsed, in neither case does it disqualify the mikveh.
Section one: If the board had a rim to it, then it counts as a vessel. Water that gathers in it is considered drawn water that disqualifies a mikveh. Without a rim it is not a vessel and the water in it does not disqualify the mikveh.
Section two: Here it is 100 per cent clear that he did not put the board under the water-spout to collect water, just to rinse the spout. Therefore, it does not count as drawn water.