Urologists often use ultrasound to perform a bladder post void residual. This is commonly done on patients who have a history of bladder infections, incontinence or a history of a poor stream with the feeling of incomplete emptying. If this value is high, it should be less than 50ccs (about an once and a half), then something needs to be done to correct that. There are surgeries for men, and meds for both the male and female howvever the technique of double voiding is a conservative way to improve how the bladder empties. The rationale and technique is nicely explained in this article from Medical News Today.
From Medical News Today