That is true about waterpumps being centrifugal but what I believe he ment is that there could be a brokenfin inside the water pump. My buddy's fullsize chevy had broken fins inside his water pump that luckily stayed right there without circulation. I don't know how else to describe it besides a fin or blade.

 

Most impellers in modern water pumps are stamped steel. They are spot welded together. Most common failure is the bearings, but if the welds are poor, the impeller can break loose of the drive hub.