Actually, that's Strunk and White's rule #17. But it's especially pertinent to learning. As Cathy Moore highlighted in her blog, there's a great study that shows that people learn more when they have to read less, and they especially learn more when they're reading summaries that use visual as well as verbal information.
Here's the big takeaway for me: Strunk and White's Elements of Style was written in 1959. The study that Cathy refers to was written in 1996. Since then, the world keeps getting faster. More and more information is available. Multi-tasking and distractions are everywhere. The need for brevity that was true in 1959 was doubly true in 1996 and is doubly true again today.
We've seen this maxim at work here at MindLeaders in the average sizes of our courses. In 1998, courses took anywhere from 4 hours to 11 hours to complete. Today, most courses are 2-3 hours, broken up into lessons that take 15 minutes or less. And we constantly hunt for ways to make our content even more quick-hitting, scannable, with plenty of ways to dig further for information if people need it.
Made to Stick is an excellent book about getting ideas across and keeping them memorable to your audience. Their very first principle is Simplicity, taking a message down to its essential core.
Concentration on the core message gives training focus and strength. What core messages are you trying to teach people this week?