The technique used here is a percussive effect achieved by dropping the side of the thumb flat onto the strings at the same time as, or just after the finger strums down. Try both ways. I’ll use an ‘X’ to represent it.

F - X t F t X t     X t F t X t F t
1 & 2 & 3 & 4 &     1 & 2 & 3 & 4 &

Once you’ve got the idea of it, play around with any variations you come up with. Some may prefer to damp with the heel of the hand – that’s fine, too. Note the difference between damping a sounding chord (and you can vary how long you let it sound before damping it) and the percussive effect of simultaneously damping with the strum. Both ways are equally valid – try them all. Learn to trust your own taste – do the things you like doing!

Sometimes a mechanically repeated pattern is a good thing, especially as part of a rhythm section – when solo or in a smaller combination a varied flow of rhythm sounds better. It depends on what else is happening, musically. If the lyrics of a song matter, don’t upstage them, support them. In trite pop ditties the lyrics are just another sound in the mix.