James Cummins (1984, 2000) explains the transference of knowledge and skills from one language to another through what he termed the Linguistic Interdependence Hypothesis.
He argues for a common underlying proficiency between a first and second language, which promotes the development of cognitive academic skills. Common underlying proficiency refers to the interdependence of concepts, skills, and linguistic knowledge found in the central processing system in the human brain. He proposes that the human brain integrates language learning.
He uses the metaphor of a dual iceberg to illustrate that although the surface features of two languages differ, at a deeper level the proficiencies developed in the two languages represent the same knowledge base.
This theory is often depicted as two icebergs representing the two languages, which overlap and share, underneath the surface features, a Common Underlying Proficiency or operating system. Both languages are outwardly distinct but are supported by shared concepts and knowledge derived from learning and experience and the cognitive and linguistic abilities of the learner.
Cummins also describes language proficiency in terms of surface and deeper levels of thinking skills. He argues that the deeper levels of cognitive processing such as analysis, synthesis, and evaluation are necessary to academic progress.
Cummins, J. (1981) The role of primary language development in promoting educational success for language minority students. In California State Department of Education (ed.), Schooling and Language Minority Students: A Theoretical Framework (pp. 3–49). Los Angeles: National Dissemination and Assessment Center.