I’ve been having some fun reading Bjork and his followers on elements of instruction. It’s good stuff! This comes from Successful Lecturing: Presenting Information in Ways That Engage Effective Processing by Patricia Ann de Winstanley & Robert A. Bjork:
In addition to its having a strong negative impact on encoding, divided attention has been shown to have much larger effects on direct, or explicit, tests of memory than on indirect, or implicit, tests of memory (MacDonald and MacLeod, 1998; Szymanski and MacLeod, 1996). The implication is that divided attention during a lecture may leave students with a subsequent sense of familiarity, or feeling of knowing, or perceptual facilitation for the presented material but without the concomitant ability to recall or recognize the material on a direct test of memory, such as an examination. As a consequence, students may misjudge the amount of time needed for further study.
Dividing students’ attention during a lecture therefore poses a double threat. First, information is learned less well when attention is divided. Second, one’s feeling of knowing or processing facility remains unaffected by divided attention, which may result in the assumption that information is learned well enough and no further study time is needed (see Bjork, 1999, and Jacoby, Bjork, and Kelley, 1994, for reviews of the literature on illusions of comprehension and remembering).