What is the meaning of 'yanking your chain'?

Back when overhead toilet reservoirs were in use, a person finishing his 'business' would have to close the lid before pulling the chain that opened the reservoir to flush. Because they were overhead, the force of the water would cause the water in the toilet bowl to splash out of the bowl. It was considered great fun to "yank another person's chain" in a public or school restroom because it usually elicited a howl of surprise and discomfort from the next stall.