I can recommend that you first watch trapcode’s form training.
https://www.trapcode.com/form_training.html
After that, download the project file to “A on Fire” and see how they did it.
https://www.trapcode.com/products_form.html
Now you should have a pretty good idea how I did it. I also used trapcode particular to create the “color and alpha” layer of the burning fist. With particular, I parented the position XY to the moving fist. Then I used the fast blur effect, this is how the color and alpha layer looks like:
https://www.ninjaeffects.com/alpha.mov
Hope that answered your question
/Markus, https://www.ninjaeffects.com