Sixteenth notes are four strums per beat: "1 e and a, 2 e and a". The arm moves down-up twice per beat, smaller and faster than before.
Two new sounds make sixteenths groove. An x is a dead strum, or "chuck": relax your fretting fingers so they touch the strings without pressing them down, and strum. You get a percussive click with no pitch. An empty column in the tab is silence: the hand still moves but misses the strings.
Exercise one, in E minor, strums the chord on beats 1 and 3 and chucks on 2 and 4, like a snare drum. On the D chord, mute and strum only the top four strings.
Exercise two groups the sixteenths 3-3-2 then 3-3-2, with the chord on the first of each group. Those chord strums are your accents, marked > above the tab; the chucks in between stay quiet.
Exercise three mixes chords, chucks and space in a funky pattern.
The most common mistake is letting the accents speed up. Keep the arm moving in a perfectly even down-up motion and let the fretting hand do the work by pressing and releasing. Start slow: 50 bpm sixteenths are already busy.
