This is also in Techniques/Strumming.
A harmonic is a bell-like sound created by lightly touching a string at a specific point and picking the string.
For frets 1-12, the strongest harmonics [they exist all along the string plane] are at frets 12, 9, 7, 5, & 4. When we lay a finger over multiple strings, lightly, and strum it, this is what we are calling a harmonic block.
Any finger can be used [directly over the fret, not in the fret space], but it can only be a single finger [more than one will create a mute].
When we use the 1 finger, the 2, 3, and 4 are free. These can mute the strings. Or, they can 'slap' or 'beat' the strings to create a thump.
It can beat or mute the strings at any point. And, any finger can be the harmonic block, while the others act as a beater/blocker. Yet, the 1 works best in our view [with 2-3-4 as beater]. Try the pinky too, with 1-2-3 as beater.
Harmonic blocks are also good 'pre-chord sounds' which can help us synchronize events to our steady strumming motor.
Synchronize the muting and lifting exactly with the strumming motor.
Try both the index and the pinky as the harmonic block. Watch the videos for exactly how to synchronize this. The up-beat isn't shown rhythmically below (we simply switch the HB and M positions).
Following these exercises, experiment with harmonics and blocking. And the beater can land in between strums to create more complex rhythms.