The water in a water-main is under pressure raised to several atmospheres. As the water flows along the pipe this pressure is gradually reduced as a result of the action of the force of viscosity until it is almost the same as atmospheric pressure at which it flows out of a fully open tap.
If the tap is covered with a finger, the flow of water in the pipe almost ceases and so the fall in pressure inside the pipe also disappears. Thus the water which passes through the opening still remaining is subject to the full pressure which exists in the water main, i.e. to a pressure of several atmospheres. The thin stream of water forced out by this high pressure acquires a much greater velocity than does water which is flowing out of a fully open tap.
When the opening is very small indeed, the velocity of the water's flow decreases as a result of the great fall in pressure at the opening itself.
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