Fallout: New VegasEdit
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 perk.
PC Xbox 360 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]
You react very quickly to the word "incoming!" Halves the damage from area attacks and splash damage.
J.E. Sawyer's Fallout Role-Playing GameEdit
You react very quickly to the word "incoming". When hit by damage from explosions or sprays, you are considered to be an additional 2 hexes away from the origin for purposes of determining damage.