A check down mark is a short memory mark.
Here is a training setup I like to run:
I think the key is to have the go bird as a long mark,
so the retriever comes back after enjoying the long run.
If your training alone with no launchers or help, you
could even start by running a long blind, then checkdown mark(s).
Then the retriever needs to focus on the short checkdown marks.
By hand throwing these marks the concept is exaggerated and
the checkdown marks are even shorter than typical in a hunt test or field trial.