I like to combo this with an overhead punch. I also use it in a 10 punch combination with cross steps and spin outs for training. Thank you for the detail.
This is one example. Here the back fists is aimed at the face. Do this same motion in the mirror and attack your reflection on the under the eye. Then use the same motion to attack your reflections collar bone and you'll feel that downward strike developing
most people think of them as quick pops, but you can get some serious power out of them so long as you know how to get the power started. In the video. Pulling back the arm starts the power generation. Do this combo right foot forward. Right straight punch - left straight punch - Vertical down back fist.
As you are pulling the left punch is extended that back fist should be at a point where it's "powered." Now try the same thing with just the back fist. No power generation.