The idea of “Stroke Contrasts” is to introduce a flaw into your stroke deliberately, then correct it almost immediately thereafter. The idea of deliberately swimming inefficiently sounds controversial, though it is done in other contexts: for example, swimming with fists (hands closed) or swimming with sneakers on.
The purpose seems to be to improve your body awareness, and thereby improve your ability at self-diagnosing stroke flaws. Even for people with access to excellent coaching (and hence external diagnoses of flaws) this is a critical skill: one can even argue that excellent body awareness is a key difference between good and poor swimmers, and yet it is a skill that is not often explicitly developed in training.
Give it a try!