This is essentially a ladle on the end of a handle. The top half of the sphere could be removed and the lower half was filled with molten metal, boiling oil, boiling water, pitch or tar. The perforated top half was then re-attached.
Shaking or flicking the sprinkler towards the victim showered him or her with the boiling contents of the ladle. The victim had of course been pinioned in advance.
Trivia & Comment
Another example of a fearsome instrument with a pleasant-sounding name – c.f. Holy Water Sprinkler.