Abstract
We show that the application of external hydrostatic pressure can reverse the ferroelectric polarization of multiferroic YMn2O5 at low temperatures. This unique effect is explained by the peculiarities of the highly frustrated magnetic order that is typical for all RMn2O5 (R=rare earth) in the commensurate and incommensurate phases. Our results indicate that the pressure changes the relative phase of adjacent antiferromagnetic spin chains propagating along the orthorhombic a-axis. With further compression the incommensurate phase transforms into the commensurate phase with the highest ferroelectric polarization.