Idle reverse basically cancelles out any "residual" forward thrust produced produced by the powerplant at the idle setting, which varies depending on the engine as installed on a specific airframe..which varies widely by type...equivalent to "ground fine/ground idle" setting on most turboprops..which tends to be more effective at higher forward speeds..many noise abatement procedures specify idle thrust only settings during landing rollout, unless required for safety considerations (weather minimums, braking action etc..) most turbojet performance considerations do not take into account the use of reverse thrust in any case..