Relaxation is one of the simplest rheological tests to describe in theory, but one of the hardest to carry out under typical ideal conditions.
The test is designed to measure the relaxation of stress of a sample following an instantaneous change in strain (displacement). A purely viscous material would instantly relax, whereas an elastic material would show zero relaxation over time requiring a constant continuous stress to hold the position with time. A more typical viscoelastic material would show a spectrum of relaxation between these two extreme, depending on the rheological makeup of the material.
In practice an “instantaneous change in strain” is not possible, and for typically rapidly relaxing rheological samples care should be taken in interpreting the results. As such, viscoelastic properties are more commonly measured using oscillation type experiments (with rheology stress relaxation measurements of academic interest to cross correlate techniques).