dp/dt means the change of pressure as a function of change in tension. which means the pressure development during isovolumetric contraction.
It's one of the ways we measure the contractility of the heart.
It follows Frank Starling Law, which states that as you increase the tension you'll increase the performance. But at a given tension it's the contractility of the heart that can increases the pressure development.
dp/dt and ejection fraction are measures of contractility not preload not afterload.
Look at Kaplan book in the heart muscle mechanics after three or four pages from the start of the chapter and you will learn more about it.