True. One of my sisters worked in a nursing home; apparently that is the way many an elderly male patient is found in the morning. The strain of moving one's bowels can create a problem for the heart, and if it's teetering on the edge anyway, that alone can be the end. Not dignified, but apparently we don't get a vote.
Basically, straining stimulates the vegus nerve, which causes the heart rate and blood pressure to drop. If they drop low enough, so do you. "Bathroom syncope" is really common, but as you said, if your heart is on the edge, it doesn't always take much to push it over.
When you smack someone in the solar plexus, you're stimulating this same nerve. It causes the diaphram to spasm, and can also cause the stomach tom spasm. So first they lose their air, then their lunch.