The player character's Critical Hit value increases +20% faster, if the player character has 250 Radiation value. This equates to 25% of the player's maximum health being filled by radiation damage in order to activate the perk.
Behind the scenes
The name of this perk 'Isodoped' is a portmanteau, combining 'isotope' and 'doped'. This is in reference to Hancock's love of chems, and 'isotope' refers to radiation and Hancock's transformation into a ghoul.