Endorphin Rush is a movement gear perk that speeds you up after you heal manually. It helps you get out of trouble right after patching yourself up.
Once you finish a manual heal, you move faster for a short time. That lets you break away from enemies and reach cover.
Good if you heal mid-fight often. It does nothing if you mostly let health regenerate.
First Aid Kit lets you heal more often, and Fight Through the Pain adds defense after healing.