Buddha, I wish I could help you out on the "whys" it works. Eggplant, as well as potatoes, tomatoes, and peppers are nightshade vegetables. All I know right now is that it is a short term aid for my wife's hypertension. I have a ways to go before I can say it's a long term cure or even effective for a long term. I am continuing to monitor her bp at least 5 times a week... and so far, so good.
It has also been said that the chemicals in eggplant aggravate arthritis. My wife has a touch of it and it hasn't made it any worse.
I have also read that Celery may decrease blood pressure, but I haven't tried that.