A lot of the discussions tend to divide sign language into either English or ASL -- manually coded English, a change of modality, on one end of the spectrum, and ASL as a pure language on the other. If that's the way to divide the pie "north and south," let me offer a way to divide the pie "east and west."
Some of my background is in Missiology. Studying how missionaries learn of culture so they can better serve the people to whom they're called, has transferred well into the choices I make as an interpreter.
Years ago, some missionaries went to the west coast "elbow" of Africa where there are many small nations. As they landed in the cities, they found that many different tribes came to the cities to do business. Although they probably all had different languages at home, the missionaries reasoned, we could work with MORE people if we learn this common language they use in the cities.
So they did.
Then, they went out to the tribes and found they were unable to relate to the people. From their experiences, missiologists developed the idea of "trade language" and "heart language." Trade language is the language of business, the lexicon has to do with economy, business savvy, and politically correct terms for conversational exchanges.
The heart language has the terms of familiarity, relational definition and development, the lexicon of person hood. The trade language could not convey the concepts necessary to start a person on a spiritual quest as the missionary would see it.
Trade language was easy to learn and more or less a "flat," uncomplicated language. Those who learned it first had a hard time with acquiring the subtleties of the heart language of any particular tribe. In the trade language, a word tended to take one path of implication. That confused the missionaries who didn't realize the same word used in a heart language situation followed a path toward a DIFFERENT implication.
Heart language inclusive of the words I loved when I was an English teacher was rich with: inference, innuendo, implication, and insinuation (and more). The deep structure of that language was acquired only when the missionary sat on the banks of the river for an extended amount of time and ate monkey meat with his tribal friends until he knew not only what they said, but what they MEANT.
Rochester, where I work as an interpreter from time to time, has some world class interpreters, those with skill to die for, but when I've asked the Deaf community which are really good ASL interpreters THEY'D CHOOSE TO INTERPRET A MARRIAGE COUNSELLING SESSION, they replied, almost none.
These interpreters in the schools and business domains are wonderful, but they're TRADE LANGUAGE ASL interpreters. Others who are not so visible and who might not do so well in a business or academic setting are the interpreters of choice by Deaf leaders, for psychiatric, psychological, personal counseling sessions, etc.; they have acquired the HEART LANGUAGE ASL which conveys concepts for mental and emotional growth in someone's person hood.
When I interpret professionally, I use the "Heart Language/Trade Language" paradigm as a template for decisions about what I'm going to try to convey. Either would use ASL features if the Deaf participant asks for it.
In a trade language situation, however, I'd choose to interpret the shoptalk, the jargon, the acronyms ("as is") and the slang frequently VERBATIM. Definitions and explanations are interpreted in a more obvious ASL structure.
In a heart language situation, counseling, for example, I think I'd choose to use other ASL terms that are more familiar, indicative of strong cultural sensitivity, and I'd alert the therapist to the fact that some implications in sign language will have to be brought out in the open in order for him or her to understand. Deaf people, just like hearing people in crisis, both want to obfuscate to hide their specific problem and "get caught," covering it up. It's the old "test" to see if the counsellor (and probably the interpreter) can be trusted enough to REALLY understand.
When I talk to the counsellor before I do the job, I may use the illustration of spelling P-I-Z-Z-A as a "pun" in sign language. In some "heart language" lexicons, when the word is spelled by making two separate Z's with the index finger, that's the food; when a person spells the Z's with both index and middle at the same time, and especially if they are bent in a curve, he means a prostitute or at least a sexual encounter. The two bent figures rather obviously represent something other than double Z's.
Can you see the difficulty if the Deaf participant tries to tell the counsellor, "I think I'm addicted to p-i-z-z-a"? The interpreter may think he or she understand the spelling, but may miss the implication by a mile!
So many times, then, wonderful ASL interpreters, far better than I, miss the "heart level" implication of those puns, even though they think they understand the words, and when the counseling process is thwarted, no one understands why. The dilemma remains for many undefinable.
I offer as a solution: many interpreters are great in ASL Trade Language, but they need to be teachable at another level of ASL, what linguists may refer to as "the deep structure," before they can manage the Heart Language of people they are called to serve.
So let's "eat more monkey meat on the riverbank."