trying to find the article I read in Racecar Engineering Mag on this.
See also
www.zeroshift.com
The keys to the kingdom is in the paragraph
secret to Xtrac IGS is the integration of a ratchet and pawl mechanism between each gear hub and the main shaft so that two consecutive gear ratios can be selected and engaged simultaneously, but with only one set of gears driving.
If I remember correctly, and I probably dont, the sifter works on the ratcheting engagement as much as gear change. You engage both first and second at the same time.
The ratcheting for second gear is engaged so it is "freewheeling" if it is spinning slower than the input shaft. When you shift to second the ratcheting for second is switched to locked and the ratcheting for fist is engaged to freewheel if its going slower than the input shaft.
Sorry, cant find the article that explains it MUCH better.