Hey, if this works in jazz music, why not in other styles of music?
To explain what a substitute dominant chord (or “tritone-sub” as it is also called – which you will see & recognize why later), take a cadential V7 – I progression:
Now, erase everything except for the tritone of the V7 chord:
If you recall, a tritone can be spelled both a d5 as well as a A4 interval. Keeping that in mind, flip the tritone upside-down. From there, you might have to respell in an enharmonic.
Finally, fill in these bare bones with what can be made as a dominant chord. As you can see, the substitute dominant acts as a bII7/I and that can be your shortcut to getting to it.
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