The effect of underloads is mostly quantified by the averaged effect on the fatigue crack growth rate, and the transient behavior is rarely investigated. The objective of this paper is to study the mechanisms behind the effect of underloads, periodic underloads, and underloads combined with overloads. A single underload smashes the material around the crack tip, producing a depression on crack flank and a local reduction of contact forces at the minimum load. The reduction of plastic elongation behind the crack tip has an immediate effect on crack opening level, which rapidly disappears with crack propagation. The smashing associated with the compressive force occurs mainly behind the crack tip position where the underload was applied. The effect of the underload is intimately linked to reversed plastic deformation, which explains its enhanced effect for kinematic hardening. The decrease of load below the minimum baseline load is the main loading parameter. The application of periodic underloads extends the effect of a single underload. The effect of the underload is enhanced by the presence of obstacles in the form of residual plastic deformation, which explains the great effect of underloads applied after overloads.

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