The Jacoby rule is a "house rule" used in most money games that encourage people to turn the cube early, by penalizing players that turn the cube "too late."
A doubling (and acceptance) voids the Jacoby rule. If the double is made and accepted, this is called "doubling in" (the opponent). If the double is declined, the result is "doubling out" (of the opponent), at which point gammon/backgammon equity becomes moot.
The Jacoby rule applies only if the cube is not turned. Then it means that no gammons or backgammons are scored, only single games,because the cube hasn't been turned. This "not scored" applies even if a gammon or backgammon is actually achieved on the board.
The purpose of the rule is to encourage people to turn the cube early enough to double their opponents "in" if they want to get the extra points for gammons and backgammons. If they wait, they always have the option of doubling the opponents "out," but such play is now rendered inferior.
What the rule does is that it takes away the "fallback" position of "Oh, I forgot to double but I can still win a gammon or backgammon if I don't turn the cube now." (That is what another poster meant by a position being "too good to double.") Because that is specifically what the Jacoby rule prohibits. This prohibition makes waiting to double opponents "out" a "second best" choice.