Today’s progression makes use of what is often referred to in music theory as a “backdoor cadence”. It’s about resolving your progression back to the major tonic, but via a VII subtonic, which replaces the standard vii diminished leading chord in the diatonic scale.
The VII chord is positioned a whole step (equivalent of two frets) down from the tonic chord. As you’ll hear in the example below, I enhance this “backdoor” tension-resolution by using a dominant 9th chord with a flat 5th, although even just a regular dominant 7th will give it the depth it needs. The result is a rather psychedelic and distant ambiguity.
The flat 5th VII7 chord is used a lot in jazz and blues, but there’s no reason why it shouldn’t be strived for in other genres and styles. A good alternative to your standard V – I or vii – I resolution.
Try out different combinations – you don’t necessarily have to resolve from the VII chord. For example – D Gm C9 Em7 D – there, the back door is left open for the ii chord (Em7 in this example)!