The original RF signal enters from what would be the far end of the TWT, where the energy would be extracted. The effect of the signal on the passing beam causes the same velocity modulation effect, but because of the direction of the RF signal and specifics of the waveguide, this modulation travels backward along the beam, instead of forward. This propagation, the "slow-wave", reaches the next hole in the folded waveguide just as the same phase of the RF signal does. This causes amplification just like the traditional TWT.