Good question.
My very limited experience is that it greatly increases the grip
on a knife. That is, if a knife had a questionable gripping surface,
then a loop over the thumb, then taught around the knuckles,
in forward grip, will make the knife very solid in the hand. In
fact, the knife would have to pivot out of your hand before
your hand could go onto the blade (assuming the cord did not
break).
So, a knife without a hilt, could be made very solid in the hand
for forward grip stabbing, by using a thumb loop lanyard. This
assumes though (IMO) that the knife will be retreaved preemptively
(i.e. you have to set the grip up), and then that hand is the
dedicated knife hand for the duration of your activites (you could
drop a loose loop pretty quick, I think, but it might also dangle
around a bit).
That's my take FWIW
Zippy