Taking 1-to-1 on your blackjack up front when the dealer shows an ace.
It is the same as insuring your own blackjack. Example: at a 3:2 table with a 10 bet, taking it pays 10, and declining pays 15 (a push if the dealer also has blackjack). At a 3:2 table decline it (take it at TC +3 or more); at a 6:5 table the payout is low, so taking it is better.