What it is
An insert is a thinner piece of cane, plastic or glass in the top of the waggler. It lets you dot the float right down and still see a lift as a fish intercepts a dropper. That is the whole point — a straight waggler with a thick tip hides the bite you came for.
Shot it so a bristle shows. Feed a line. Fish on the drop or at dead depth. It is the traditional waggler, refined.