It is not clear what the meaning is of the Hebrew word which has been translated in this verse as water shaft.
The general consensus is that it refers to a tunnel which connected the city to the spring of Gihon.1 Presumably David and his men entered the city by means of the water shaft and this then allowed them to have a surprise attack and conquer it.2 Possibly, if his men did not enter the town through the shaft, they could have cut off the water source and thereby forced the town into submission.3
8 And David said on that day, “Whoever would strike the Jebusites, let him get up the water shaft to attack ‘the lame and the blind,’ who are hated by David’s soul.” Therefore it is said, “The blind and the lame shall not come into the house.”