Getting back to basic science of schooldays, the action of centrifugal and centripetal – same principle is applied here. For example while doing pawanmuktasana where you hug your knees close to your chest the additional pressure exerted on the body is a positive pressure. The blood from the concerned places gets squeezed away to other places this is known as centrifugal.
Negative pressure means vacuum like condition is created in the body or parts of it. Example while doing Nauli the abdomen is drawn in. Observe Baba Ramdevji doing it miraculously. Here due to the low pressure in the concerned part, blood from the neighboring parts of body is drawn in, which also is known a centripetal.
Hence it is advisable to perform Yoga on an empty stomach and early morning is the best time as the bowels are cleansed and the system is light. It is important and very beneficial to have stomach and bowels cleaned up because in many of the practices these parts are contracted and stretched or positive and negative pressures are created on them and thus the blood circulation is affected.