The tilt table test is done to evaluate fainting episodes (syncope). Syncope may occur due to:

vasovagal—a postural drop in blood pressure with or without a decrease in heart rate that is caused by a dysfunction of the nerves controlling the heart and blood vessels
abnormal heart beat—an abnormal heart beat resulting in lower blood pressure.
The tilt table test is designed to simulate the syncope or fainting. During fainting what happens is the question answered in this test: does the heart rate increase or decrease; does the BP increase first or HR increase first?
Depending on the answer of these questions, one can sub classify the type of Syncope and plan treatment of the patient.