That re-sets the dalek targets on the mini playfield. When you have the playfield in the up position, it attaches under the playfield on the right side of the mini playfield.
iirc, you take off a nut and use the pre-existing bolts/hardware of one of the other assemblies (mini pf?) put this on and bolt it back down with the nut.
You can see it assembled and on the playfield in this picture here:
(as the pic is rotated, it is on the "top side" of the mini playfield in this pic)
http://www.phoenixnestarcade.com/blog/wp-content/uploads/2008/09/dscf0063.jpg