Well actually it's a matter of low band apply and shift timing. In the drive position the rear band is not applied in first. If you pull it into first this applies the rear band. While that sounds well and good these days, in the earlier days timing the release of the rear band and the application of the front band was a problem and created a momentary drag when shifting. This already happens in the 2/3 shift when releasing the front band and applying the front clutch. Eliminating the rear band apply in first gear provided a cleaner shift.
