I think we tend to classify things "internal and eternal" base don what we THINK is going on,
A big, wide smacking slap is external while a short, seemingly effortless slap is internal.
Maybe, maybe not.
Here is how I see the internal/external thing:
If a movement is over reliant on muscles and less on technique then it is eternal ( think a beginner haymaker), but when the same move is based on correct structure, alignment and proper muscular effort at the correct time ( maximum result with minimal effort) then it is internal.
My 2cents.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !