Turn the cylinder upside down and see that the water flows down with difficulty. Now, rotate the cylinder fast. After a few seconds, a funnel shaped cavity is formed at the centre.

What did you observe? 
Water starts gushing down.

The science behind it :
Water rotates faster at the periphery than at the centre of cylinder. As a result, centrifugal force act outwards the water column. Consequently, the surface opens up by forming a cavity at the centre. The air from bottom escapes through this cavity and then the water flows down. This cavity at the centre of whirlpool is known as ‘Vortex’.