A Brain Functional Controller is a part of the brain which controls sentience, action, and reflexes. If it's removed by the Hypnotic Memory Gun or the Kiss of Death, the brain parts left behind still operate, but will have no use because the Controller is the main part.
When removed, the victim becomes a Zombie, while his/her real self is locked away inside their own mind, in a state of loss and confusion, unable to speak, feel or move for him/herself, but only think. He/she would be trapped forever unless the Controller was put back.
The Controller can only be placed back in the victim through one dangerous medical procedure (which is the only way [This chance is lethal if failed or miscalculated]): Brain surgery. There is only a 2% chance that the surgery would be successful & that the victim would not die, The other 98% chance is that there will be a miscalculation that results in failure of successful brain surgery, causing brain, heart, stomach, kidney, bladder or liver failure & the victim would die. Kowalski agreed to take the risk & his research says that almost 3 out 21 penguins survive brain surgery. 18 out of 21 penguins die in brain surgery.