I will demonstrate the topic in Am Scale but it could be in another scale.
in a lot of songs,music there is a chord progression that includes these to chords in this order : Am ---> Dm and musicians especially jazz musicians use pulling chords/passing chords in between these two so it sounds more effective.
In my musical life and thanks to experience I have discovered two :
1.using A7 :
Theoretically it is good because it's the fifth of Dm - this one sounds good and beautiful and I guess the scale becomes D harmonic minor in this pulling chord (it sounds like this).
2.using Gdim7 (diminished 7th):
This one sounds heavenly touching and more emotional and I don't understand why it sounds great (it would be great if someone can explain me why it is working).
But sometimes I hear even a more harmonically touching pulling/passing chord that I wish I could get to know/discover but I'm not able to.
So,can you tell me what pulling chords you know that can be used in this harmonic progression? It would be great if someone can explain me the why the Gdim works well.