early in my xingyi studies, our teacher sat us all down and explained to us how our strikes were different from typical hitting, and why we were called an internal martial art. He had us put hands around his abdomen as he moved a "ball" of qi around his abdomen. Then he had us all feel the "electricity" above his hand after he channeled qi to it.
Then he said how our internal strikes don't just do external damage to things, but penetrate the muscles and damage the internal organs behind them. He had us all stand up and form a line. He then proceeded, one by one, to put his palm on each of our abdomens, have us tense up our abs as much as possible, and then fajin (a 0-inch palm strike). every one of us immediately doubled over and almost puked. ALMOST. the bile rose to our throat, and then stopped right there. Everyone ALMOST puked. no one actually puked.
One of my shixiong under the same teacher, was talking to another friend of ours who was learning kendo from our same teacher (in addition to many years of shaolin and xingyi, he was also kendo 5th dan, judo 3rd dan, TKD 3rd dan). My shixiong said that our palm strikes could penetrate muscle and destroy internal organs. The other friend was incredulous and said, "No way. Your strike could not penetrate my kendo armor" (the do, abdomen, part). My shixiong said yes, it can. so it was on. That friend donned his Do from his kendo armor. My shixiong set up, his palm against the armor. He then warned our other friend that his control was not as good as our teacher's so he may throw up. Our friend laughed and said he can go ahead. My shixiong inhaled, and then BAM, fajin into the armor. Our other friend doubled over and puked in his own mouth, but luckily not more than that. he believed us from then on.