Having only a limited amount of RAM accessible, you simply had to rewind the tape drive in order to process data. (E.g., for the UNIVAC I, the first computer to rely on tape drives, there were the commands 1nm, 3nm to read/write forward and 2nm and 4nm to read/write backward. That's also what the seek-command in any file-API is for.) The vacuum columns provided the extra amount of tape to control the tension and keep the tape from snapping. In the animation, the exaggerated action of the tension rollers stands for this functionality of the vacuum columns, which can't be shown in the limited screen estate.

(In the ambient audio there's a "floop" like sound, probably produced by the vacuum columns on occasion of a reversal of the direction.)