Moving protons in water cause so-called "flow artefact" on T2 weighted images since they are not refocused by the 180 degree spin echo pulse.  This artefact can be useful to determine patency of tubular structures such as blood vessels, aqueduct of Sylvius and in this case to confirm that a left paracystic mass is connected to the bladder via a flow void of urine passing between the true bladder lumen and a diverticulum.