his test determines the severity of any incontinence and objectively demonstrates the symptom.

Urine produced in the kidneys is transported to the bladder by rhythmical contractions of the ureters.
At appropriate occasions the bladder is emptied via the urethra, by contraction of the normally relaxed detrusor muscle that constitutes the wall of the urinary bladder.
The detrusor muscle's behaviour is thought to sometimes become 'unstable', leading to the lower urinary tract symptoms – such as problems with frequency, urgency and getting up at night to pass urine.
An unstable detrusor also contracts between voidings. This causes high pressures in the bladder that may be felt as urgency and give rise to urine leakage.
In men, many of these same symptoms can be caused by enlargement of the prostate gland (BPH) and urodynamic studies can help to differentiate the two causes.
For obstruction, the International Continence Society has agreed that presently the only way to objectively diagnose it or grade its severity is a urodynamic pressure-flow study.
For incontinence the issue is more complicated. There are two main types of incontinence:

1. stress urinary incontinence (SUI) that's caused by a deficiency of the closure mechanism of the bladder
2. urge incontinence that's caused by overactivity of the bladder. This overactivity can be demonstrated urodynamically by filling cystometry.