Reciprocal inhibition is a muscle relaxing when its opposite contracts, sometimes used to help the release along.
In this library it is a working picture, not settled physiology as a medical claim and not a diagnosis of which muscle is inhibited. 📝Resisted Soft-Tissue Release may ask for the opposite muscle so the target can let go. If it does not, the picture was not useful that day.
Clients feel the other side working. That can be enough.