I think that this is what he means:
If the mother has their ability in slot 1, then the baby has an 80% chance of hatching with that ability, and 20% for the second ability and viceversa if the mother has the ability in the second slot.
-So if you put the mother in slot one of the daycare, there is an 80% chance that the ability will be passed down and a 20% chance that it wont.
-If you put the mother in slot 2 there is a 20% chance that its ability will be passed down and an 80% chance that it wont be passed down.
However, this does not work if you use a ditto. It is still a 50/50 chance to get either ability.
But do note that this is only in B2/W2.
For hidden abilities, it does not matter which slot they are in. If the mother is in either slot and has a hidden ability there is a 60% chance that the hidden ability will be passed on.