When I first come across the words "poison arrow", my limited English does not give me a clue until I looked at the author's picture that it was in fact a house situated on a "T". Simply put, a poison arrow is something like an arrow shooting at you, and it goes without saying that something shooting at you is bad or poisonous.
There have been a lot of talks among realtors and those who know little feng shui that a house situated on a "T" is bad, and perhaps very bad, and should be avoided. However, like every theory, there is exception.
To the layman, it is better to avoid "poison arrow". However, to the experienced and knowledgeable geomancer, it is not bad to have a house situated on a "T" provided the geomantic calculation is correct. I always encourage people to learn feng shui themselves because there is no certainty as to whether you have met a knowledgeable geomancer who would like to help. However, like every discipline, you must have the time and the financial resources. I have come across many westerners who think that they can buy a few books and paid a hundred dollar and then they will become a feng shui master or geomancer. I was very surprised at their wisdom of thinking because nothing comes cheap. One cannot buy a few cheap medicine books and become a doctor. One also cannot buy a few cheap legal books and become a lawyer.
To return to the effect of "poison arrow", if the calculation is bad, it is indeed very bad, and is indeed life threatening. Therefore, think seriously if you are not proficient in feng shui (a little knowledge is a dangerous thing by Alexander Pope so the poet says) and you want to live in a house situated on a "T". However, if the calculation is good, it is indeed very good, and will bring in a lot of luck depending on your fate. The speed of good or bad effect is often at lightning speed and therefore be very very cautious.
For the non-feng shui master who normally have the kind of building structure on a "T" coincidentally happens to Kings and Queens, the rich and the powers, e.g., the Windsor Castle, the Buckingham Palace, the House of Parliament in Canberra, Australia, to name but a few. This kind of building structure has a grander view for those living in the house and on the opposite side of the fence, a longer and broader area for the spectators to look at the house.