Split attention effect

This article is about an observed effect in learning, i.e., a claim (backed by observation and experimentation) that some kinds of actions in the context of exposition, learning, practice, review, etc., on average, cause learning outcomes to move in a particular direction.
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Definition

The split attention effect is an effect seen in poorly designed instructional materials where comprehending the material requires constant toggling between two different modes of presentation, thereby hindering comprehension and cognition.

For instance, a geometry example that requires constant toggling of attention between a diagram and a written explanation might cause the split attention effect. Similarly, an example that requires people to switch attention between a video presentation and written text might cause the split attention effect.