[vunderba]

One of the points in Input Hypothesis is something I'm sure everybody would agree with:

This states that learners progress in their knowledge of the language when they comprehend language input that is slightly more advanced than their current level. Krashen called this level of input "i+1", where "i" is the learner's interlanguage and "+1" is the next stage of language acquisition.

Informally I've always called it "walking the knife's edge" - you have to be always on the slight edge of feeling uncomfortable to realize meaningful gains. I mean it makes logical sense. The brain is ALWAYS trying to optimize away through chunking/patterns/etc. so you have to be constantly challenging it.

It's the reason why there's a huge skill difference between a driver at one month vs 1 year, but a far less difference between a driver at one year vs ten years.