The concept of RDS is extremely simple! The cartesian co-ordinates are modified to represent distances in terms of powers. The x-axis represents the position of far-point and near-point respectively. The y-axis is used for indicating the power difference between far and near-point (basically the distance between far and near point on the RDS). If you're curious where accommodation is factored in RDS, it's basically proportional to distances observed closer from the far-point up to near-point. And remember, power is just inverse of distance.
To begin with, I will first represent an emmetropic eye on the RDS.As a known clinical observation, an adult emmetropic eye can focus from infinity up to 25 cm at the very least. (failure to focus at 25 cm without experiencing eye-strain is usually taken as indication of Presbyopia onset)
An emmetropic / pseudo-emmetropic eye on the RDS
If this emmetropic eye develops Myopia later and requires prescription of -1 D to see distance objects clearly, the 'naked' myopic eye can be visualised on the RDS as given below.
It should be easy to visualise how a shift of one unit towards left (wearing -1 D lens) enables this myopic eye to see at infinity once again like an emmetropic eye. (we term it pseudo-emmetropic because it behaves like an emmetropic eye only after wearing prescription)
A myopic eye requiring -1 D prescription
For those who are further interested, here is what an emmetropic but presbyopic eye requiring 1.5 D ADD represented on the RDS (notice lower y-distance than the graphs shown above).
I think it will be trivial for anyone to explain why this presbyopic eye can't see at infinity while wearing its prescription for close-up.
A presbyopic but emmetropic eye
Edit: The simple 'confusing' intention behind introducing y-axis in RDS.