Although Brain Salad Surgery is now associated with Emerson Lake and Palmer, and Doctor John, I remember the expression from my first days in design. What it referred to was pulling out the insides of something very complicated, completely changing it, and getting it back in and having the whole thing work again, fixing some problems along the way.
It was used to describe the process to correct a problem that was caused by the basic design concept. So even though the unit under test looked the same when you were done, the entire "brain" had been removed and replaced, with all of the implied connectivity complexity.