So for example if a person complains of a high frequency of urination it might be because of detrusor overactivity. Urodynamics would involve the use of pressure transducers attached to catheters inside the bladder and rectum to verify such a pressure pattern.
Detrusor overactivity is characterised by the phasic pressure contractions seen in the image. This is not a normal finding. The normal bladder should be low pressure during filling.
External link: [Doug Small's urodynamic pages http://www.sghurol.demon.co.uk/urod]