I don’t understand it fully yet but here is my guess:
In the walkthough about transformers (4) Jeremy explains that you use the class Transfromer or the decorator Transformer. This in combination with a Type annotation in your function (or encoder/decoders function of your class). If you do this it will check if your input is the right type and if so it will apply the function on that part of the tuple.
Not sure if this all still makes sense. Please do look at walkthrough 4. It is really good and made me understood it (I think). The linked post is also quit a thorough explanation.