a tape transport mechanism that senses the end of a tape while minimizing but not eliminating back tension at the tape - feeding end . excessive back tension causes uneven transport in fast forward or rewind . insufficient back tension causes slack that makes sensing the end of a tape impossible . both result from keeping the torque identical on both takeup and feed reels . the mechanism automatically switches the torque as the tape reverses so that the takeup reel always has a higher torque than the feed reel , thereby insuring that the tape end is sensed and that tape transport is even regardless of speed .