These auto-reversing tapping heads allow the machine spindle to rotate in the forward direction without the need to stop and reverse, two times for every tapped hole. We call this Constant Speed Tapping.
With Constant Speed Tapping, reverse occurs almost instantly when the machine retracts. By eliminating the deceleration, stopping and reacceleration of the machine spindle, the cycle time truly follows the actual program speed and feed rate.
If you have a lot of holes to tap, and cycle time is important, this is the way to do it.