I still cannot put a value on the core writings of AJATT by Khatzumoto. His original methods are highly effective and people everywhere are using them even today. They are encouraging and guiding. The concept of i+1 is something Khatz talks about freqeuntly in his writings, but never explicitly explains. Why? Because it's a very simple concept that makes sense on paper, but feels difficult to implement at first.
The i is what you know. Your current understanding of your target language.
The +1 is one small extra thing that you don't know or would have to look up.
Applying this to sentences, you want to find sentences where you understand every word, particle, etc. plus one thing you don't understand. Armed with a dictionary, you can fill that void and acquire that new word in your mental vocabulary.
Throwing these i+1 sentences into your SRS (Anki) for later review allows you to retain these new vocabulary in your memory.