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 David avait dit en ce jour: Quiconque battra les Jébusiens et atteindra le canal, quiconque frappera ces boiteux et ces aveugles qui sont les ennemis de David… C'est pourquoi l'on dit: L'aveugle et le boiteux n'entreront point dans la maison.