This review presents experimental studies on improved confinement (IC) modes observed mainly in stellarator plasmas. The IC modes result from structural bifurcation, often accompanied with discrete and sudden changes in electromagnetic and thermodynamic structures. The phenomenology of the bifurcated states occurring in various stellarator configurations is described with the fundamental experimental observations associated with the mechanisms of bifurcation and confinement improvements. Particular emphasis is placed on the physics of radial electric field, which is a key quantity that leads toroidal plasmas to structural bifurcation. Neoclassical (-driven) transport barriers commonly observed in stellarators are highlighted.