I - V/3rd - VIm - IV: pop/rock ballads width descending bassline
Major chord with the 3rd as bass tone
A little change in just one chord of the I V VIm IV progression leads us to the typical sound of rock and pop ballads that you know from countless hit songs.
In the major chord on the 5th scale degree we play the 3rd as the lowest note instead of the root. Like this a G major chord for example becomes a G/B chord (say "G over B").
Chords with a written bass note different from the root are called Slash Chords.
Write: chord / bass note
e.g. G/B or F/A ...
In this video I'm playing the progression through all 12 keys (with just some of the chord variations shown above):