The condition that only five-card hands of 8 or lower qualify as a low.
It qualifies if all 5 cards are different ranks and the highest is 8 or lower, and the ace counts as 1. Example. 8-7-6-5-4 is the weakest qualifying low and A-2-3-4-5 the strongest, but 9-6-4-3-2 does not qualify because the 9 is too high. If no low qualifies, the high takes the whole pot. In Stud 8 you pick any 5 of your 7 cards, so there is no two-hole-card restriction.