Reduce blood pressure will induce the brain to send a signal that constricts the blood vessels.
Baroreceptor will catch the impulse that saying there is decreased blood pressure and send it into the neuron that will end up in the spine. It is located in the upper body so the signal should reach the brain.
The brain will send a signal to constrict the blood vessel of the body, but the signal couldn't pass the lower body. This will cause the lower body failed to receive the signal, thus not constrict the vessel. Therefore, the blood will be pooled in the lower body and the blood pressure keep decreases