In the map source, the only important triggers are triggers *2 and *4.
Trigger *4 constantly activates itself and sets variable [p_hp] to the player's amount of HP.
Trigger*2 constantly activates itself and compares the current HP ([p_hp) to the player's HP that was before that ([player_hp].) If the current HP is less than [player_hp] then it will continue the trigger's actions and play a BNG sound. If the hp is not less than [player_hp], then it wont.
Basically, the 2 triggers check both the player's current HP and the HP before it got changed (or the player's current HP if it didn't change), and checks if there is a decrease.
The other mechanism (which is the right one) is a simpler version (AND CONTAINS ONLY THE ESSENTIAL STUFF) and checks the Usurpation.