The immersion style of teaching a language in the purest sense involves refusing to use other languages to aid in teaching the target language. So if you take a French class in France, you might not hear a word of English. Whereas if you take a French class in the US, some teachers will speak English at least in the first few stages.
I find the immersion approach extremely slow and error prone. E.g. if the teacher holds up an image of a red firefighter hat and speaks French, you might not know if she is saying “hard hat”, “red”, or “fire fighter”. You have to guess and if your guess is wrong it feeds into negative training.
There is an audio tape where a Brit teaches French. He said for the most part English words ending in “…tion”, “…ly”, “…ize”, “…ise”, etc are also French words. There are exceptions of course but in just one sentence of English I instantly learned hundreds of French words trivially.
Not sure how thoroughly this has been studied but I suspect immersion language teaching works better on quite young (highly neuroplastic) brains. As an adult it’s very frustrating.
A professor once told me: you don’t need school to learn. You can learn anything by teaching yourself by reading and experiencing the knowledge. But what school does for you is accelerates the learning by structuring it for fast consumption in an organized way. I agree. And I think that the most accelerated way to learn French is to use existing knowledge of English as a tool. Whereas learning by immersion is comparable to learning by experience (the hard way is slow!).
So my ultimate question is whether this as been studied on adults. Does an adult group reach fluency quicker or slower if they learn by immersion? A lot of people say immersion is more effective, but it always seems like this guidance is blind. They never say or imply it’s supported by research. It seems like an indoctrination that people just accept. Different brains are different. An adult who only knows one language will probably be more hindered by immersion because their brain perhaps relies more heavily on associative memory (making connections with existing language knowledge).