Important regulatory events in both prokaryotic and eukaryotic transcription are currently explained in terms of an inchworming model of elongation. In this model, RNA extension is carried out by a mobile catalytic center that, at certain DNA sites, advances within stationary RNA polymerase. This idea emerged from the observation that footprints of individual elongation complexes, halted in vitro at consecutive DNA positions, can remain fixed on the template for several contiguous nucleotide additions. Here, we examine in detail the structural transitions that occur immediately after the enzyme stops at sites where discontinuous advancement of RNA polymerase is observed. We demonstrate that halting at such special sites does not "freeze" RNA polymerase at one location but induces it to leave its initial position and to slide backward along the DNA and the RNA without degrading the transcript. The resulting loss of contact between the RNA 3'-hydroxyl and the enzyme's catalytic center leads to temporary loss of the catalytic activity. This process is equilibrated with enzyme return to the original location, so that RNA polymerase is envisaged as an oscillating object switching between catalytically active and inactive states. The retreated isoform constitutes a principal intermediate in factor-induced endonucleolytic RNA cleavage. These oscillations of RNA polymerase can explain its apparent discontinuous advancement, which had been interpreted as indicating flexibility within the enzyme.