A hold-up after a king (from K-Q) lead towards an A-J combination that forces the leader to switch or concede a trick to both the ace and jack. For example: you are South in 4♥ and West leads the ♠K. You should use the Bath Coup and duck the first trick. West cannot continue spades safely. Now, when East gets in with the ♦K, you will be able to win the second spade and throw the ♠J on a diamond.