Learner control principle #1: Give Experienced Learners Control

Clark and Mayer (2011) on page 319 of our textbook recommend giving learners the option of going beyond the essential content, or giving them the option to skip segments of instruction that they may not need to review. To implement this suggestion it is necessary for the designer to cue learners as to what parts of the instruction is necessary for completion, and which parts can be skipped by either novice learners or by experienced learners. This is valuable when the instruction is intended to be used by students with different backgrounds. Otherwise, novice learners may become discouraged by segments of instruction that are difficult and not essential to their instructional purposes. And experienced learners should not be forced to plow through material they already know.

Source: Kahn Academy -- Linear Regressing and Correlation

In the example shown when a learner begins to view this part of the course (a 7-minute video) he or she is told that the mathematics is relatively difficult and that it is not necessary to view this particular video in order to continue and complete the course. In other words, this is made available for students who what to "drill down" on the mathematics behind a specific concept. Without such guidance, students may be unable to discern what content is essential (to complete course objectives) and what segments can be skipped. Without that information, novice students may become bogged down in material that is not necessary, and experienced students may not realize that a particular segment is likely to be of special interest to them. 

References

Clark, R. C. & Mayer, R.E. (2011). E-learning and the science of instruction: Proven Guidelines for consumers and designers of multimedia learning. San Francisco: Pfeiffer.