Ex-showdown equity is the money won (or lost) from from future betting. Showdown equity is the money you win if the hand were checked to the river with no additional betting.

Ex-showdown equity is transferred whenever a player folds with non-zero showdown equity (i.e. folds the best hand or a hand that has a chance of improving to the best hand), and the amount of the loss is equal to the amount of the showdown equity, for example the entire pot if the best hand is folded on the river. Ex-showdown equity is also transferred when a bet is called at an ex-showdown equity disadvantage. This means whenever you call a bet with a low showdown equity hand before the river when there aren't enough implied odds or bluffing opportunities on later streets. It also means whenever you call a river bet with the worst hand.

In your example it is correct that you should call with only 2 combos of the bluff catcher. Defensive strategies on the river need only make IP indifferent to betting with his strongest bluff. In many cases this will be a complete air hand but not always. It actually pays quite a bit in OOP play to de-polarize IP's range so he has to bluff with pairs for this reason.