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For a cassette tape, the tape moves at a rate of 1 7/8 inches/second. The tape is pulled across the tape 'head' when a 'pinch roller' pinches the tape between itself and the 'capstan'. The capstan is entirely responsible for the speed at which the tape travels. The take up reel spools the tape back onto the reel after it passes through the capstan and pinch roller. The takeup reels are driven through a clutch which allows some slippage. It can not be driven at a constant rate of speed because the diameter of the tape on the reel is constantly changing. When the direction of tape travel is reversed, the previously active pinch roller is pulled away from the capstan, allowing the tape to pass freely by the capstan. The pinch roller which was previously not active, is pushed against the capstan. The capstans rotate in opposite directions. The reel table which previously was just spinning free now becomes the 'takeup' reel and the other reel spins freely. The diagrams below show the operation for each direction of travel.
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