Flower nails have two purposes, one is so that you can spin it while frosting a flower making it easier. Another purpose is to insert into a cake, helping to evenly distribute heat and cook a cake more evenly.
The word "sift" derives from "sieve". In cooking, a sifter is used to separate and break up clumps in dry ingredients such as flour, as well as to aerate and combine them. A strainer is a form of sieve used to separate solids from liquid.