Welcome to Just Answer and thanks for your question. When people sit in one position (or stand in one position) for a time, blood pools in the legs, causing dilatation of the vessels and decrease in blood pressure. Normally when they stand up the body causes the release of epinephrine (adrenaline) to cause the heart to beat faster and harder in order to pump more blood to the brain.
When someone has neurocardiogenic syncope, there is a disconnect in the messages. Instead of getting the word to beat faster and harder, the heart beats slower and the blood vessels dilate further. This causes an abrupt drop in blood pressure, less blood flow to the brain, etc. The person feels lightheaded, dizzy, and may faint. Fainting is actually a good thing in this case because the person usually falls or is placed with the head on the same level or lower than the feet, and blood flow to the brain is increased. After an episode of syncope, the person usually feels fatigued and their mental abilities aren't as sharp as usual.
It is one of the reasons a person on a plane should get up and walk about every hour. When sitting, it's good to clench and unclench your toes and wiggle your feet and ankles occasionally. This helps the blood flow back toward the heart (and the brain).