The exact equations of motion for the oscillating two‐stream and parametric decay instabilities (OTSI and PDI) are derived in the reference system co‐moving with the ion‐acoustic disturbance and their physical interpretation in terms of wave pressure is presented. It is found that OTSI and PDI behave differently: whereas in the OTSI the ponderomotive force is shifted by −&pgr;/2 with respect to the disturbance, in the PDI case it is in phase with the propagating ion wave, as in stimulated Brillouin and Raman scattering. The decisive role of the pressure of an electrostatic wave is stressed.