What is the function of the sperm mid-piece?

Answer 1

The sperm is made up of three pieces, the Head, the Mid-Piece and the Tail. The head continas the important part, the nucleus with the genetic information to be passed onto the child. The tail, which is actually a Flagellum, provides a rapid waving motion which pushes the sperm cell through the fluid allowing it to "swim". This obviously requires much energy which the cell gets from the sugar Fructose not glucose like most cells. To get the most energy out of any sugar requires it to be fully metabolised (oxidised) to just CO2 and water. To do this the cell used organelles called Mitochondria, it is not true that only the Ovum has these as some would say but it is true that only the ovum conributes them. There are 6 motochondria stacked on top of each other in the midpeice which provide just enough energy to keep the cell going. Once the head penetrates the ovum (assuming it is the lucky cell) the mid piece and tail drop off and are no longer of use.

EDIT: While the above answer is very good, I believe this is what you were looking for.

The sperm cell consists of a head, a midpiece and a tail. The head contains the nucleus with densely coiled chromatin fibres, surrounded anteriorly by anacrosome, which contains enzymes used for penetrating the female egg. The midpiece has a central filamentous core with many mitochondria spiralled around it, used for ATP production for the journey through the female cervix, uterus and uterine tubes. The tail or "flagellum" executes the lashing movements that propel the spermatocyte

Answer 2

the mid piece is a bundle of mitochondrions and is the actual motor of the spermcel