The fuze was set so the balls would be ejected when the shell was on the downward part of its trajectory over enemy troops. The fuze is still attached to the top cone of the shrapnel shell which was ejected with the fuze to allow the balls to depart. The shell had a small gunpowder charge in the base to push the balls out.

Heres some more reading on the subject-
http://www.army.mil/cmh-pg/faq/shrapnel.htm