Switch Spawning Killifish are simply species of Killifish that can be spawned in either the Plant Spawning or Soil Spawning methods. They are just more adaptable. You can try either method with the particular species you are working with and see which method gives you the best results.
Note that when the Plant Spawning method is used the incubation period is usually much shorter, about 2 weeks. But when the Soil Spawning method is used the incubation is longer, up to 6 weeks. The Soil Spawning method can come in handy if you want to collect a lot of eggs and hatch them all at the same time.
Most, if not all, Killifish can be spawned and their eggs collected and placed on top of moist peat moss patties, even the strictly Plant Spawning only types. This sometimes comes in handy if you are having a lot of trouble with fungused eggs. Though with the Strictly Plant Spawners the incubation period remains about the same as normal (2 weeks).
The Blue Gularis is an example of a switch spawning killifish.