The ulnar nerve goes to finger 4 and 5, and the median to 2 and 3 plus a branch to 4. For me, 3+4 is the most difficult combination to maintain, and raising 4 is the most difficult, but the effect (as you suggest) is strongest in the non-dominant hand.
So I interpret this position as simply the most difficult hand position to maintain, thus indicating some intention, practice, or awareness -- and thus self-control, which was considered the master virtue classically.