Adrenaline Surge (5e Class Feature)
From D&D Wiki
If you are missing hit points when you enter your rage, you can choose to gain a number of temporary hit points equal to your missing hit points. These temporary hit points behave differently from other temporary hit points. Any damage you take applies to your current hit points first, then the temporary hit points granted by this feature. Once you are at 0 hit points, you only become subjected to any effects that occur when you are reduced to 0 hit points once the temporary hit points run out.