Watch the 16th Note Displacement Combos: 4 online guitar lesson by Jon Finn from Rhythm Lab: Rock & Funk
Since grouping 16th notes in groups of 3 creates a 3-measure pattern, each example takes out two measures and demonstrates 2 two-measure patterns. Example 19 is the 1st and 2nd measures of Example 15. Example 20 is the 2nd and 3rd measures of Example 15. Because you're playing a "partial" pattern by only playing two measures, there's a rhythmic "skip" in the pattern that makes it interesting and funky. But this time, it takes longer to get to the "skip".