As far as physics is concerned, an external blow is a physical fracture or cut. A fifty caliber (.50) aircraft machine gun bullet; will kill you without contact, if it passes within two inches from your skull, without touching you, due to the concussion and/or hemorrhaging.
Hung Gar is mainly based on a skeletal-linear leverage of muscle tendon based movement. Its damage is usually concentrated on bones and the surface. The exception to this is those who have bee shown the advanced levels of 'Iron Thread' (uncommon).
Hsing-i was taught with a wave motion: that emphasized the flow cumulative flow of whole body motion that is leveraged with its unique scissor stepping.
What is now commonly taught as Xing Yi, might well not be any different than Hung Gar.
'After all these years observing martial arts I find it difficult to explain how Hung Gar is "external" and Xingyi is "internal", for example.'
A test of an internal punch is not moving the sandbag, but in focusing the blow so one does not break one's own wrist with a strong strike.
A true internal test is to hit a concrete or brick wall without breaking it or your hand. A bell ringer uses an internal concept.
As far as an internal blow is concerned, the best documented case was:
'First Demonstration of Kuo Yu Chang in 1925, Kuo Yu Chang's iron palm abilities were witnessed by a certain Hwang Hsien Sheng. To summarize the story, a Russian circus had posted an open challenge to anyone who would dare take three kicks from one of their horses. Anyone who survived would receive $1000 in gold, a huge sum of money at that time. Kuo Yu Chang accepted the challenge under one condition; instead of money, Kuo asked to strike the horse with one slap of his palm. The Russian owners of the circus accepted his conditions. In front of a huge crowd, the horse raised his hind leg and kicked Kuo in the chest. The crowd was silent in disbelief. Kuo then gathered his strength and when the horse kicked Kuo a second time, the crowd roared. When the horse kicked Kuo a third time, the crowd gave Kuo a huge ovation. Kuo then rested for more than half an hour. When he returned, he struck the horse in the rear, and the horse fell dead. Again, the crowd cheered at this incredible feat.
The famed Eagle Claw master, Lau Fat Meng, witnessed the postmortem on the horse. He observed that there was no external wound on the horse but that inside there was a large bruise on the horse's back and some of the horse's internal organs had been badly damaged .' From:
http://www.jadedragonalaska.com/iron-palm.php
An external blow would have cut the horse's body or broken its back; it did neither.
Drop a small firecracker in a swimming pool, and it the water's wave internal action will crack the pool. An external blow, would require dynamite.
Back in medieval times, most city people rarely traveled as much as twenty six mile in one direction. The curvature of the earth is not apparent unless over twenty six mile, so in Galileo's time, for most people, the world was flat.
For those uneducated, un-experienced or unskilled; there is no internal.