Manually Coded English (MCE) is basically a system of English codes on the hands, using some ASL signs and adapted ASL signs. It uses English word order and grammar based on English.
MCE is an umbrella term that includes S.E.E 1 (Seeing Essential English), S.E.E. (Signing Exact English), and later Signed English. They emerged in the 1970s after realizing the failure of oralism during the "Dark Age" between the infamous Milan of Congress of 1880 and the 1960s.
In the 1960s, Dr. William Stokoe of Gallaudet Unviversity discovered the linguistics of American Sign Language, which shows evidence that ASL (as well as other signed languages) is indeed a language. Later in the 1980s or so, neuroscience studies show that the lingustic regions of the brain show similar activities in both aural-vocal and visual-spatial modalities, which means that language is amodal.
Back to the point, Signing Exact English (SEE) is a manually coded form of English. It's not a natural language. To help you see the analogy, imagine that you're learning French (or another language that you're not familiar with), you only learn French vocabulary, nothing else. Just vocabulary that you're good at. Then you speak French with English grammar. That's what SEE is alike. Although, there is more than just vocabulary. Processing (from the phonological level to the semantic, syntactical, and social-linguistic levels) is also involved in a language.
That is, S.E.E. and other MCEs can corrupt language acquisiton and education in deaf children. This is an analog to forcing children to write English (grammar) using spoken/written words of another language (e.g. French, Arabic, or any). In the end, these children often struggle.
"It goes to-me good." What? It's a transcription or glossing for the German phrase, "Es geht mir gut." which means "I'm doing fine" in English. English and German have their own structure, grammar, and culture. The same goes for ASL that cannot be Englishized. It's also important to note that it's okay to make grammatical errors when learning a new language, which is part of the learning process.
In the 1980s, ASL courses flourished across North America for hearing students. But, signed English communication became widespread in the classrooms of Deaf students. Total Communication (TC) and Simultaneous Communication (simcom) were used in Deaf students' classrooms. Lingucism and audism persisted, unfortunately.
In the 1990s, a study presents that 90% of teachers of the Deaf used TC. And, 80% of teachers of the programs for Deaf children used an MCE system.
The results in those past decades show that education for Deaf children had not improved. And, audism persisted.
Deaf children of Deaf parents often outperformed deaf children of non-signing hearing parents in language acquisition, literacy, education, leadership, and so on.
"Nativization hypothesis" is that all children have an innate mechanism for the acquisition of language through language experience. MCE can disrupt nativization of language learning. It inhibits or limits the development of language especially a second language, English. With a strong full-fledged language acquisition in ASL from birth, children have no problem learning English as a second language. Same true for everyone, who has a first or native language, can learn another language, regardless of signed, written, or spoken language.
Linguistics and neuroscience research and studies recognize that ASL is a natural, real language. Bilingualism is clearly the best approach to education of the Deaf. Today it's a norm in many Deaf schools. Through bilingualism (ASL and English) in education, Deaf students attain education and literacy on par with hearing students.