Your extensive familiarity with Explosives allows you to ignore a portion of their damage. Your Damage Threshold is increased by 25 against any and all Explosives—even your own. This makes Hit the Deck one of the best perks in the game for a character who makes heavy use of Explosives, especially with all 3 ranks of Demolition Expert and the Splash Damage perks active.
pc There is a display error with this perk. The +25DT will be shown in your PIP-Boy menu whenever you have an explosives-category weapon equipped. However, the added DT will correctly only apply against attacks from explosives-category weapons, regardless of what you have equipped. [verified]