Phoenix-eye fists, dragon's head fist, panther/leopard fist, Lama's fist, the Sean Connery Presidio Thumb, etc,etc.
Force multipliers come in many shapes and sizes, no pun intended, and with them come a varied of training philosophies that can be summed up into to categories:
Forging and non-forging.
Some believe that one must forge/condition the tool, others are of the view that conditioning (at lest high level forging) is not needed, relying on "pinpoint precision" for the results.
I am of the forging school you can say.
As it is, I train/forge my PE fist and because I use a "thumb on top fist", I train my thumb knuckle too.
Typically I forge them on the IP bag and the hanging bag, also on the Heavy bag and some special drills on the medicine ball and hanging dummy.
And the makiwara of course, along with (for the PE fist) push-ups.
As for their use, I have used them a few times in sparring, though my partners tend not to like that, no one likes the feeling of knuckles into ribs and shoulders.
I tend to use them a bit in the ground work context, especially when in the guard.
Again, controlled and not full force ( I can drill the PE fist ALMOST at full force at this stage of development).
I have only started forging them as of the last year and a half.
Why do I bring this up now?
Good question....