The P-d effect occurs as a result of lateral curvature being induced in an axially loaded element. A parabolic moment distribution is induced along the length of the element which alters its effective stiffness and causes the curvature to change further.
P-d effect
Unless the axial load P exceeds the element's critical buckling load, a point of equilibrium eventually occurs such that the P-d moments are balanced by internal flexural resistance built up within the element.