When you look at a blurred object your brain moves the focus of the eye in and out until it gets an increase in contrast. If no increase is achieved your brain leaves the object unfocused.
Pinholes spectacles improve the sharpness of the image, allowing your brain to find the point of focus.
During this process the focusing muscles of the eye are getting used more and this regular exercise helps your eyes' focusing system.
JD Harris pinhole glasses help your eyes focus and in so doing exercise tired eye muscles. In turn your eye's focussing ability improves.