I can't get my head around why. I mean surely you are just moving when you strum to a half second or so later. I mean, you start the beats when you want. So surely the beats and half beats are just whenever you want them to be. So why does it sound different? I cant see what differentiates a beat from a half beat.
Actually you don't start the beats when you want -- or at least you shouldn't. The beat is inherent to the song and it's melody. So, no matter where you strum, the 1 is always on the first syllable of 'worry' in the line 'don't worry about a thing' and likewise on 'thing' for the next bar. If you were to strum on 1, 2, 3 and 4, your 1st strum would be on '
worry' and on 'thing'. If you strum on the 'ands' it would be on 'wor
ry' and after 'thing'. All subsequent strums move up one quaver as well. So all strums align differently with the lyrics, which is why it sounds different when strumming on the 'ands'.
It's a little hard to explain, I guess, but even if you strum on the 'ands' you should still be able to 'feel' that the bar starts on the 1 only your first strum comes a quaver after the bar started. Does that make sense?