I'm no expert on this, but I'm currently reading Steve Silberman's Neurotribes, which is a cultural history of autism. It's not specifically about homosexuality, but it does delve into the use of electro-shock therapy on autistic children, as what's known as aversion therapy. Basically, the psychiatrist gives an order, and if it isn't followed, a shock is administered. Theoretically, the subject will learn to avoid the behavior on his/her own.

This article from the Huffington Post (but written by an archivist at the National Gay and Lesbian Archives) Silberman proposes that those administering these "cures" believed they were acting humanely. Given that it was deemed impossible to change the attitude of society toward homosexuality, it was more humane to change the undesired behavior.

This article (from the National Gay and Lesbian Archives) might also shed some light on your question. Remember that homosexuality was considered a mental disorder before 1973, and the prevailing attitude of psychologists seemed to rest on curing disorders, instead of either encouraging acceptance or studying how best to integrate into mainstream society.

The 'principle' behind electroshock therapy, as I understand it, is to use electric shock to regress the mind to a state of blankness, creating a "blank slate" to impress the desired state on using repetition of words from the therapist or on a tape. The goal is to rebuild the psychological make up.