Dick is right.  Although not easily noticeable in the picture, the sling was utilized around the right thigh!  The text says some riflemen were still using this position, which was allowed for 800 and 1,000 yards (although most riflemen were using the prone position).  The back-position was more awkward with the Krag because of the bolt handle and sight distance.  It was popular with the .45-70 Springfield.  The toe of the Butt-plate in contact with the ground allowed Mother Earth to absorb much of the recoil.  (It was the practice of many Target Shooters to thread their sling through the rear barrel-band/middle swivel and attach it to the front "stacking" swivel).