The way my cardiologist explained is that the baring down like when constipated has the same effect in causing a vagel response which can trigger a heart attack , he also said that it's generally from people with poor heart health ,the average healthy person should not have an issue..
Apparently Drug addicts that are addicted to pain killers are prone to it as well from the obvious issues they cause with the heart but also the pain killers relaxing the paristalsis in the bowels making them constipated.
I know even small things like doing a kagel like you are trying to stop the flow of pee can have a noticeable effect of lowering blood pressure and heart rate enough to throw off a baseline on a polygraph.
In situation when people pass out after a big deadlift or squat it's mostly from the blood pressure rising during the exercise then drop when they take a deep breath
Same when you stand up and get dizzy , it's the sudden drop in blood pressure