When you have 3h2h on a 4h 5h 6h x x board.
When you have KK on a AAKKx board.
When you have 75 on a 34689 board.

In all these cases you have the second nuts, but in the first hand opp can't lay down Axh, in the second hand he can't lay down Ax, in the third hand he can't lay down a set. If he has 8h7h, AA, or T7 respectively, then we pay him off.

In general, when you have a hand that is better than what you're opponent plays the same as the absolute nuts (or the old "you're ahead of his range") .