When a so-called "mictic stimulus" ("Mixis stimulus" in German) occurs in the environment, which could be for example changes in the chemical composition of the surrounding water or the temperature, the females may produce haploid eggs. If these eggs are not fertilized they develop into haploid males which are usually pretty much smaller (there are exceptions, for instance Asplanchna) and look different from the females (sexual dimorphism).
These males are pretty fast swimmers. If they get in contact with a mictic female they can now fertilize the (principiclally the same) mictic eggs . These fertilized eggs develop an embryo and are (then) called "resting eggs".